I used to lease a horse that got super sweaty, too. I think it has a lot to do with genetics. Like humans, some horses just get sweaty more easily than others.
I was worried about my horse sweating until my trainer reassured me that that it's not a big deal. If you're worried about your horse being overworked, then watch for flared and bright red nostrils. Sweating isn't always a good indicator.
I'd say if your horse isn't obviously sick or in pain, then he probably just sweats more than most. If you're really concerned, you could always asked your vet the next time they're out.
From the sounds of it, your horse is in pretty good shape, so I'd be pretty willing to bet that he's just a sweaty horse. :)