I've dipped a lot of seahorses, most successfully, some not.
A freshwater dip will tell you if there are parasites or not as the seahorse will thrash around for a while and sometimes you can see the residue of the parasite in the water after the seahorse is removed.
For freshwater dipping a seahorse, match the temperature and the pH of the tank water, placing the seahorse in the water for 12 minutes. ONLY remove the seahorse sooner if it becomes unresponsive when you touch it.
Have you tried live food yet?
It is much better if you transfer the seahorse to a hospital tank so it can be viewed closely and see if it eats when you don't look but placing a numbered amound of mysis in the tank and checking the numbers later in the day.
Get the temperature down to 68° or as close as possible to lessen the chances of bacterial outbreaks.
By the way, there is hardly any LFS's out there that know anything significant about seahorses except how to make money selling them, and some do poor at that.