There are many devices that can scan and read. As with all assistive technology, the key is to find the right match for the individual user. You know your child better than anyone.
Another great example is word prediction. So for students who have difficulties with spelling, we have these fantastic programs that are very much like the auto correct on your smartphone.
As you noted, there are a variety of typing programs on the market. The best answer to your question is that there may not be one single program that best meets your son’s needs.
I think the AT Matrix has last documented more than 400 assistive technology tools out there on the market. So finding the right tool can be challenging.