Easiest, and most likely way to fix this problem is to have him neutered.
When Coal was 6 months old he started humping people, we got him an appointment and had him neutered ASAP and it stopped completely!
No, he's not in any discomfort or pain, his hormones are going crazy because of his age and it's causing him to hump as a reaction, he probably doesn't even understand WHY he's doing it!
I'd make the appointment now and get him neutered ASAP if you want him to stop humping