My son has ADHD but we didn't know that until he started school. He is my only child and I thought he was just energetic. My son had delayed speech and didn't potty train until he was four years old. He never had melt downs to the level that you describe. When you put your son in his room during a melt down do you put him by himself? I would think it a good idea if you or your husband were in the room with him if only to speak calmly and repeat that when he is calm he can leave his room to do a desired activity. My son's pdoc said don't ever restrain him when he is upset. We did have success with time outs although he would often be upside down in the chair during the time out but in the chair in any position was okay.

Do you have a routine for meals, baths, bedtime? Routine is very important. Also let your son know before an activity changes that it will end in ten minutes so he can mentally prepare.

Will he drink smoothies? You can sneak lots of fruits and vegis in them.