Does he sweat in small areas or across his body? My mare used to sweat a tear shape patch on her offside hind quarter all of the time. My equine physiotherapist told me that in her case, it was a sign of a trapped nerve. After several months of treatment in the area the patch dissapeared. It was really weird though and the vets took loads of pics and showed every trainee vet they had on placement!
My friend's Arab x gelding used to sweat on one half of his body...so in warm weather or after work....it started when he was in his early teens I think.
The vets did not seem concerned about it, just told my friend to make sure she washed him off so his skin was clean.
It would seem that they were right - he is still in work and is hitting 30 now.
S