This seems reasonable for his size but a lot depends on the horse's build/body style. Some are very substantial and others are more fine-boned and that can have a big influence on weight. If you can feel ribs then you're probably OK, unless he has a crest or fat pads on his butt and along his ribs. Weight tapes are not that accurate for minis when used in the traditional way, there was a post recently that had some links to calculators that might be more accurate, or just Google "horse weight calculator" and a bunch pop up. You'll need the heart girth measurement plus the body length. By the calculator, my 37" guy is right at 300lbs, he is fairly substantially built, and I can feel back ribs easily but the front ones are a little more buried.
There are also other reasons for round bellies besides being overweight - he could just be well-sprung, could be a hay belly, worms, lack of muscle, etc.
Also right now, I see you are in MA, we have had a major fluff-up in the last two weeks! So it could be at least partly hair...