when I see a belly like that, my first question is- what are you feeding? the belly looks like an malnorished baby(bloated stomach) as you've seen rescue groups on tv for. He needs more and the 'right' nutrition. I saw 6+ horses at the last show look like this and then I saw what they were feeding. ugh. Start from the beginning and go from there. one little medicine isnt going to cure him and let a vet look at him, could be something else going on in there too. But you need to change your feeding program. when I got Jr( my yearling) he looked horrible, pot belly, ugly dull coat. So for example- Twice a day- my yearling is getting one cup dry beet pulp(soaked), 2 cups crimped oats, 6 oz alfalfa cubes, 1/4 cup Enrich 32, daily wormer, 6cc body builder and a handful of loose nice grass hay.. he is a bit chunky at the moment, but he has a 13 hr drive to World in a couple days and he will drop about 10 lbs during the trip.
I just had to say this, I think when I was scanning down, I saw you said you were using Omolene? then build him up to- use 2-3 cups Omolene , 1/4 cup Enrich 32, 2 cups crimped, rolled oats.(just a runned over oat). 6-8 oz alfalfa hay or better yet cubes, you can weigh them. and start with a cup of beet pulp and move him up to 1 1/2 cups beet pulp measured dry, but soak it. do this gradual change in about 5-7 days, write it down so you know what the schedule looks like and get him on a daily wormer, it looks expensive at first, but a healthy horse is a happy horse and a lot less trips to the vet. Another thing, people will say - well this isnt a show horse- so then what, you are supoose to feed them crap, do attorneys and doctors eat different high quality foods than a blue collar worker, NO, we all must eat healthy, no matter what you do. Good Luck