Sounds like classic founder and since he is a high risk age and breed, it is extremely likely this is the case. You would have to cut back his feed tremendously and he can't eat any of that lush grass for some time, at least a month and maybe more (probably won't hurt if he is overweight anyway). Diet has to be changed immediately before there are any changes in the pedal bone as this is irreversible and can happen surprisingly quickly.
The only symptom that doesn't sound like founder is the dragging of the back feet. How does he stand in the stall? If he is lamanitic he may appear to be leaning back on his hind legs to ease the pressure he is feeling in his front feet.
All horses deserve, at least once in their lives, to be loved by a little girl.