Many answers here seem to me excessively pessimistic. Rather than commenting on them individually, let me make some general points:

Yes, language has changed since Medieval times, so knowledge of the modern version of the language would be of only limited value. But it's absurd to say that this would mean that the people of the time would think him insane or a drooling idiot because he can't communicate. Medieval people were well aware of the existence of other countries where people spoke different languages. There would be nothing surprising about such a person at all.

The fact that he is an "outsider" would not result him in instantly being killed or thought to be a witch or any such thing. While travel was limited in Medieval times, there certainly were people who travelled great distances. Ever hear of Marco Polo? Travelers were usually welcomed because they brought news from the outside world and entertaining stories. A traveler could usually get a few free meals or at least some drinks by telling a few stories.

Yes, health and sanitation back then were poor, so the risk of an early death was greater. But it's not like everyone was condemned to instant death. Obviously many millions of people managed to live to adulthood. Maybe you could make a case that there would be diseases circulating then that are not common now, and so he wouldn't have any resistance. 

At the other extreme: 

It is very unlikely that he would be able to bring much if any modern science and technology to this place. Sure, he'd be familiar with computers and cell phones and airplanes and all sorts of modern things that these people never heard of. But could he teach them how to make these things? I doubt it. I like to think I'm a smart guy -- Mensa-level IQ, work as a software developer, and all that -- and I'm hard pressed to think what technology I could introduce if suddenly dropped in Medieval times. 

Yes, I can use a cell phone. But I haven't the vaguest idea how to build one. 

I know a lot about computers. But build one from scratch? Let's see, microchips are made using silicon and germanium. I know silicon comes from sand. Exactly how do you extract it? Where do you get germanium? I presume that lithium batteries are made with lithium. Duh. Where does lithium come from? What does it even look like? I don't even know what color it is. Even if I had a pile of lithium, how do you make a battery from that? Etc.

Okay, maybe that's too ambitious. How about something simpler. Maybe a steam engine. I know it has something to do with boiling water, having a small opening for the steam to come out, and running that steam into a piston to make it move along a shaft. But how to make any of those things? Maybe given the time and resources to experiment, I could figure it out. Or maybe I'd just kill myself when my first experimental boiler exploded.

Thomas Edison once said that genius is 1% inspiration and 99% perspiration. Even if you know the basic idea behind some invention, there are many, many details to be worked out to take it from a "basic idea" to a working model, and many more to get from a working model to something that is actually practical. Unless you are an engineer whose job it is to design models of this particular machine, the odds are that you have no idea what the details are.

Many inventions build on earlier inventions. Before you can make a radio, you need electricity. To produce electricity, you need things like wires and magnets. To make wire you have to smelt metal. To smelt metal you have to mine raw ores. To mine raw ores you have to know what the ore looks like and where you find it. Etc.