A false percula clownfish (Amphiprion occelaris) is a compatible tankmate for a seahorse providing you are willing to feed the pony properly to make sure he gets his fair share at mealtime. But I suspect the clownfish would be more likely to fulfill the role of chief competitor for the frozen Mysis rather than becoming a good buddy or close companion of your seahorse. Your pony may prefer the solitude to a pesky dinner guest with a bottomless appetite.
If you are concerned about your solo seahorse becoming lonely by itself, you might consider taping a mirror up against the aquarium glass where your pony can get a good look at himself. Seahorses will often interact with their own reflections in the aquarium glass, so having a mirror-image seahorse that moves in response to his own actions can be very reassuring for a single seahorse and perk up the isolated individual dramatically. It’s an effective technique for a situation like yours and can fool the lonely seahorse into thinking he or she is still in the presence of other seahorses.
In short, I think I would give the old mirror trick a try before I introduced a chowhound clownfish that may wear out its welcome all too quickly.
Best wishes with all your fishes, Nigel!