From what you've written, your child has essentially been a shut-in. You need to give him the ability to socialize, not so much time with flash cards. I see this as an emergency situation. No child should get to age 5 spending all his time with a parent or caretaker. Academics will come-- address his social needs and overall happiness first.
ETA: I realize that you were trying to finally address the need for socialization through early entry to K, but the problem is that you let him languish for far too long, with predictable results. Just as you wouldn't expect a puppy to entertain itself all day with a ball inside a cage while you work on your business, your son has real needs that you have not met up to this point, and that has to change.
What's done is done, and you have to just do your best to remedy the harm caused as well as you can. Immediately get him into as many interactive situations with other people outside the home as much as you can. He may be extremely unsure of himself in any social setting you find for him, and you will have to be creative now in finding ways to help him deal with that. This may take some work, but it's necessary to undo what's gone before.
