I forgot to ask an important question: What species of seahorse do you have?
Also, I can see two things that you can change right now for sure to help him out:
One, assuming he is a large seahorse and not a dwarf, try switching him over to frozen mysis shrimp. Frozen brine shrimp doesn't have as much nutrition to it and is high fiber, and since seahorses have a very simple digestive system, they barely get anything out of the brine before it passes through their system. If he won't eat frozen mysis, the next best thing is to get him on enriched live ghost/glass shrimp (freshwater or saltwater, whatever you can get), and then once he is eating well again, try to move him over to the frozen mysis. Getting him off the brine is especially important, since when he isn't eating much, everything he eats needs to be packed with nutrition and good fats.
Two, assuming he is a tropical seahorse, bring the temperature down slowly in the quarantine tank, and in the display tank, to 74 degrees or less. Don't decrease the temp by more than 4 degrees in 24 hours. In the quarantine tank, it would actually be best to get the temp to 70 degrees or less since he seems to be suffering from something. The bacteria seahorses are most succeptible to are more aggressive and mutate into more dangerous strains at temperatures higher than 74 degrees, so you want to keep them lower. If your seahorse is a subtropical or temperate seahorse (whitei, breviceps, potbelly, etc.), you are going to want to slowly bring the temperature even lower.
Also, try to get your husband to tell you what the water parameters are. It is good that there were no fluctuations, but a small amount of ammonia or nitrite over a long peroid of time can be just as dangerous, so you want to double check that both ammonia and nitrite are at zero in both the display and the quarantine tank. I know I am not supposed to mention other web sites on this web site, but in your circumstances, I really feel like you will have a better chance of saving your seahorse if you post a picture and tank details in the diseases and treatment forum on seahorse.org, there are a lot more people on there that have helped people through treatments like this.