Foal with big belly?????

I used to work for an equine vet that specialized in breeding. There were quite a few orphaned foals in our care each year, and it was always the orphaned foals that bloated like that. From what I understand, it's a side-effect of malnutrition from lack of mother's milk. It's not always caused by worms. I would definitely talk to your veterinarian about it. If he's on a good de-worming program then a change in diet might be required. Our orphaned foals always grew out of it with some decent healthcare, nutrition and exercise. A bloated belly may also be caused by lack of exercise. Younger horses who were on stall rest, or who had been ill and inactive for some time would look like that. There are many possibilities here, so consulting a veterinarian is definitely your best bet for finding answers. I would just advise against de-worming him until he's confirmed to have worms. Too much de-wormer can do more harm than good. You don't want to de-worm a horse out of schedule before you know what he has. Certain de-wormers only kill certain species of parasites, so even if he has worms and you blindly de-worm, you may not fix the problem.