Because of the thick slime coat on all jawfish even if he does have blackspot, it's going to be near impossible to treat with medicine. I have heard that jawfish are particularly resilient to copper medications, but I'd take that with only a grain of salt.

Even if he does have blackspot, the best treatment may be to stay on top of water quality, make sure he isn't being harassed, and make sure he gets a vitamin and protein enriched food.