What species seahorse do you have? Assuming it is a tropical species, I'd recommend the following:
Ad an airline to the tank that he is in, and gradually bring the tank temperature down to 68 degrees by no more than 2 degrees every 12 hours. Watch him closely to make sure that he does not get worse, and hopefully the 68 degree temps will take care of it. If he does start getting worse, or if a few weeks at 68 degrees with good water quality and excellent feeding don't remedy the problem, come back and we can recommend meds for a hospital tank. If he gets better, keep him at 68 degrees for a few weeks, and then slowly raise the temp to 70-74 degrees; and keep the display at this temp permanently.
If the tail is floating and the bubbles are clear, it is likely that he is suffering from external gas bubble disease, which has a variety of causes, two of which are high temperatures and excess CO2; which is where the lower temps and airline come in.
If the tail is not floating but seems sore, and the bubbles are opague, it may be a bacterial infection; in which case the extra airline is to reduce the stress of getting oxygen at the high temps/increase oxygen saturation, and the lowered temperature stops/slows the growth of the bacteria so that the seahorses immune system can fight it off. The bacteria that seahorses are most prone to mutates to produce more aggressive proteins and reproduces at a higher rate at temperatures above 74 degrees; often 68 degrees can hault the reproduction altogether. And, seahorses are definately prone to bacterial infections due to being scaleless, without protective slime coat, and demersal.