My (now 16 year old) son has been doing this since he was 4. I believe it's been the single most positive influence on his physical and mental conditioning during that time. With the proper instructor and determination of a student, there are no limitations. Great story!
Good for him. No matter what health condition you are in. Nothing should stop you from doing anything. I have trained people with various types of limitations and they progress very well.
I believe I heard of Tokey Hill before. I do recall him doing Goju-ryu Karate.