Could he still be fertile?

No, not if he's been neutered. A Vet once told me that dogs hump to show dominance or because it's an ingrained habit.

I have had female dogs that humped other dogs, toys, etc.

And I have a smaller female who humps the larger male, dominance is a strong issue for dogs.

We have 4, 70#, 55#, 35# and lastly 10#.
Our 10 pounder occasionally humps the larger ones, they are all spayed and neutered and she
Has done it for years.
It's is likely a dominance issue and should not present a problem unless aggression ensues.

When was he neutered? A neutered male still has potential to impregnate for a short time after the surgery (3 weeks I believe?)

Past that, it is probably a dominance/behavioural thing, as mentioned in the above posts.

I agree with the others. This is a dominance thing. All dogs want to be alpha, but few are. Still, most dogs will try to assert their dominance, and humping is a part of that.

Correct them right away... its dominance and if they do it to the wrong dog they will get bitten. A couple months of correcting and they should stop.