First, you realize pens are inherently portable. So the first step is to make the typewriters small enough to be easily portable. With a genius-level foresight (or a dumber than dumb 21st century hindsight) it is clear that mechanical typewriters will not quite fit the bill - electronics is the way to go, especially the newly discovered semiconductor devices.
Something like this, replacing the hard to operate mechanical construction with an electrically operated printing ball and an easy to type keyboard s an improvement:
Still too heavy. So then comes the next model:
That's better, but for quick notes, paper is still cumbersome, big and heavy (in reasonable quantities). Replacing the paper with a built in display is an improvement:
It could be made smaller, but then typing would become more difficult.
The next step is to make these (first luggable, then portable, or perhaps even pocket) typewriters to be widely accepted and used by the population worldwide. Granted, with prices getting low enough, these will be used, but mostly by enthusiasts and typewriter geeks. So, in addition to writing down notes (which is the main use of a pen), what can our typewriters offer as their killer app? Well, we people are social beings, so the ability to communicate remotely... while nothing new under the Sun, making it ubiquitous and portable (and with some far reaching imagination, even wireless) would have some appeal...
So, after some iterations, your typewriter fits into a pocket and either has a reasonable keyboard, but a bit heavy:
through smaller, but still very usable:
to the poor man's typewriter with a limited keyboard, requiring multiple presses of a key to enter the desired letter:
And who knows where the future development leads, but one thing is clear, the devices will have a keyboard - although we can imagine technological development to allow us other input methods, who would want to write on anything that is slower, error-prone and clumsier than a keyboard?
That's for the two way written communication... but can we do better? Obviously, N-way (where N>2) is better - imagine a worldwide network, with hundreds, nay, thousands of discussion groups, where
you can type you opinion, read the responses from your peers on your pocket typewriter screen, even communicate in real time... I can imagine this to be the major timesink and a new most indispensable thing since the television. And once this becomes widespread, people will use handwriting (and pens) less and less and eventually even the school system will catch on.
And, if the development starts with the transistor and continues in its natural leisure pace, prompted now and then by generous investments in the right spots by your entrepreneur, and if in the 1950 he is young and healthy, with a bit of luck he might live long enough to see his dream come true.