Keep giving him access to good food and let him catch up as quickly or as slowly as his body needs to. I know its hard and you feel like you need to be stuffing food down his throat to get him to grow, but slow and steady is the way to go. I've been battling the same thing with my stunted yearling filly, Kenzie. She was pulled out of a terrible situation where she was alone, not being fed whatsoever for an unknown amount of time, wormy, thrushy, and very, very sick. She wasn't even 12.2hh when we brought her home, and 14 months old.
I was worried about her growth and didn't think she'd ever be tall enough or healthy enough to be ridden, but I followed the advice of my vet and a few experienced forum members here and provided her with an aquedate amount of vitamins, minerals, protein, starches, and fats...and she started putting on weight. It seemed like she'd never grow for a while, but once her body had repaired itself on the inside, it started to grow. She isn't growing FAST, but she is definitely growing and I can tell she has a lot more growing to do. Her string test tests her at being around 15.2hh when its all said and done which I thought was impossible at first, but at 22 months old she is now 13.3hh at the wither and butt high, so I'm beginning to think she just might make it to be at least 14.2hh...normal horse sized ;) small for a Thoroughbred, but very acceptable. She's gone from looking like a 5 year old to looking almost her age (nearly two) in just 8 months!
14 months old
22 months old. Excuse that she's slightly ribby...darned filly eats more than her 16.2hh pasture mate, but she's putting all that food into growing, not filling in her ribs!
Your colt is still young and hasn't done the majority of his growing yet. Give him good food and he should grow up just fine. Does he have access to pasture? Allow him to eat as much good quality hay (a nice grass hay is always good, but alfalfa can be fantastic for youngsters in need of a little boost too) as he wants, then if he's looking a little thin still, give him a good youngster feed like Nutrena Mare and Foal (that's what my filly eats) or Triple Crown, or even just a ration balancer if you don't think he needs the extra weight but does need the nutrition. I've heard great things about Enrich 32.
As Foxhunter said, don't over feed him, as that can cause bone growth problems and make him more susceptible to injury later. Lots of hay and grass, a little bit of Ration Balancer or Youngster feed, and I'd say you're good to go.
I'd get a vet to do a fecal test on him and worm him appropriately though, too. A pot belly isn't 'normal' with a young horse, although it is seen a lot because they can carry worms so easily.