Did he have some kind of traumatic experience that led you to believe he needed therapy? If not, then I am not really sure why he is in therapy from what you've described. Just because a child is 5 and doesn't always listen well doesn't mean there's anything wrong with him - he's just 5! Kids that age are supposed to be active, and along with that comes not listening (sometimes), not always following directions the first time, not being able to sit still - that's just the age, and with consistent direction/correction, he will improve his behavior.
Send him to Kindergarten. He is the right age, and if he is reading already, then delaying his start in Kinder would be doing him a great disservice. Believe me, he won't be the first antsy boy the Kinder teacher has ever seen!
Try not to worry so much about whether he is happy. Children do not need to be ecstatic 24/7 in order to be generally happy. Sometimes in the course of providing loving discipline, we will make our children unhappy. That's just life, and they will get over it! Normal kids cry, pout, get mad, laugh, play - they go through the whole range of human emotions, and that is totally normal. Just let your little guy do his thing, he will be fine!