This is actually a standing question in philosophy. The connection between the sense and thinking is the topic of many essays, and language is typically considered to be sub-topic within "thinking." As such, there will be no easy cookie-cutter-answer. There never is, when it comes to philosophy. Instead, there will be subtle questions that try to dislodge your concept. The idea is that, by exploring the answers to these subtle questions, you come closer to actually understanding what you were originally asking.
In this case, I am going to take your two definitions for the subject's state, and play with them a bit:
The only way in which the world can communicate with him is through brain signalling. So they can send different signals to his brain, which will evoke an distinct emotion in him.
The only way he can communicate back with the world is through brain signalling too. So he can send a specific kind of signals back.
Now let's implement this. We're going to need something wire like which can send electrical (or chemical) impulses directly into the subject's brain. Presumably these are connected to machinery that are designed to convey "meaningful" stimuli to the subject. In this case, I define "meaningful" to mean "has meaning for at least one person," which may be a doctor in the room, the subject itself, or even a deity, if one believes that deity exists. The exact definition is left intentionally vague, but it explicitly excludes the idea of a content-less world. It seems unlikely to me that, in a world devoid of content, language would ever be useful enough to develop.
Now we're going to need a way to signal the outside world. Again, we need something wire-like to take electrical (or chemical) potentials from the brain and convey them elsewhere. Now for this step, I'm going to assume that another individual exists, and that the equipment is designed to help convey meaning to them. While you can find meaning in a solipsistic universe where you are the only conscious entity, communicating with language in such an environment is strange. So let's assume someone else exists, just to hand wave away that tricky bit. Presumably that device measuring brain states would be built in a way to maximize the possibility of meaningful transfer of information.
Okay, let's implement this a bit further. It can be really hard to pick up highly transient signals unless you know their meaning ahead of time. They're a difficult class of signals to act on. So let's simplify further. We're only going to signal this brain via enveloped sinusoidal signals. This means that each signal has a frequency, and its amplitude changes over time much slower than the beats of the frequency itself. I like this subset of signals because, as an engineer, I know I can build software and hardware to work with them. For example, I can take nearly any transient sequence, and via a windowed Fourier transform or a wavelet transform, and I can break that sequence up into a bunch of these enveloped sinusoidal signals. This means the engineers are happy with you when you hand them the plans for your machine. These engineers may also point out that some of those signals can be hard enough to work with, that it'd be convenient if they could drop the sinusoid bit and merely convey the amplitude information. It moves much slower, so it's easy to work with.
Now for the signalling out of the brain. If I was developing a way to help someone communicate out of their brain, I'd concentrate on approaches which permit exploration. No brain is going to learn to output perfect 10/8b encoded digital messages with CRC-32 checksums. It's too hard, too little room for exploring ways to use it. Instead, I'd try to create an analog system. I'd try to create something where the response to the electrical stimulus for the brain is continuous. I'd also try to do something which makes feedback-based-learning easy. Since we have an enveloped sinusoidal signal providing inputs, it'd be really helpful if the output of the brain was also an enveloped sinusoidal signal. In electrical engineering, we have a great tool for this, called a Voltage Controlled Oscillator (VCXO). When you change the voltage, the frequency of the output changes. We also have a bunch of great things called filters, which can take a signal, from a VCXO or any noise source, and filter it based on frequency to "shape" the sound. We could hook up our subject's brain to these devices, and then let them experiment with them. Maybe give them a stimulus and see if they can replicate it.
Now I just picked one very small subset of your test space. You were wondering if you could communicate with the subject through brain signalling. I made the question far harder by narrowing it down to a single implementation of that brain signalling. Obviously, if I can prove that language is possible in my case, it will prove that it works in yours.
I also chose my example pretty careful, so that it is easy to prove. My input system parallels human hearing, where the cochlea breaks down sounds into different frequencies before transmitting their amplitude information into the auditory cortex. Likewise, my output system parallels human speech. Voice synthesizers for computers actually use these exact inputs. You provide a signal with frequency information plus noise, and then filter it into the sound you want. The real human vocal tract has vocal chords to provide those signals and noise, and we contract muscles to shape the mouth to emphasize and de-emphasize different frequencies (if this sounds strange, check out Tuvan Throat Singing, where they intentionally shape the mouth to resonate a harmonic of the note being sung, rather than the note itself!)
So, given that my subset of your problem describes the approach of a hearing and speaking human individual, and they learn language, that should demonstrate just how versatile the concept of language learning can be!