The cause of weak snick is unclear, and its likely caused by many things, not limited to parasites, but anything that might damage the trigger mechanism. A piece of sand or rock could do the same. In my last go around, I believe it was physical damage, because force feeding and time were what it took to solve the problem; I did not treat for parasites.
Based on what you've described, I would start tube feeding if it was my horse. However, that's something only you can decide and there is some potential risk to it. I've never actually had a problem with the procedure itself, but it is always possible.
I wrote up a detailed explanation of force feeding here:
http://www.fusedjaw.com/advancedcare/instructions-on-sedating-force-feeding-and-cpr-for-seahorses/3/
You may also want to read page one and two which covers basic equipment and sedation techniques. I also advocated using clove oil, which I still use, but if you can get your hands on ms-222 (finquil) it's a better sedative.
If you don't go the tube feeding route, I'd try getting some live mysis asap. A lot of seahorses just refuse live brine. Its not natural for them, and it doesn't trigger the right response. IME, H. comes rarely takes live brine, no matter how healthy they are. I've seen H. reidi refuse it too, thought that's more 50/50. You'll likely have to order the mysis, SeahorseSource has it, as does Sach's aquaculture store. If you get lucky, you may be able to find it in the refugium of a local reefer or the live rock tank or refugium at a fish store.