It sounds like he's suffering from "weak snick." I don't know the preferred method of treating this, but I think there is some information on
www.seahorse. org under the emergency or disease forums.
If it's already gone on for 2-3 days, you need to do something quick. Your horse may already be getting past the stage where it could recover. If it were mine, I would move it to a quarantine tank with 100% new water (of course matching his current PH and temp), and offer it the smallest live shrimp I could find.
I think your first clue to his disease is going to be in his home. You don't say how many horses you have, but a 12 gallon aquapod is not large enough for even 1 horse. If this is where you have your herd, this could very well be a heads up to an upcoming disaster. I would give very serious consideration to setting up a minimum 29 gallon tank for just 1 pair of horses, larger if there are more.
If this is not an immediate option, then other options, such as selling them, trading them to the LFS for credit, etc should be considered, because you will not be able to keep them for long in that size tank anyway.
Good luck!