
I'd be trying to distract him from the humping then rewarding him when he stops, and also trying to catch him when he's about to do it and reward him for stopping before he does - that's the preferable thing to reward of the two as well as it'll help his self control :-)
Neutering can make zippo difference to them being a sex pest - Remy was a sod his whole life, he had River numerous times each season she had and Paige a couple, and Willow's first season was horrendous - by that point he physically couldn't mount her or even try because of a slipped disc, so he whined and barked his way through it instead. He was neutered at 10 months!
Is your boy confident generally? Excessive marking and humping can both be signs of nerves rather than hormones (marking - trying to look confident, humping - not sure what to do but I know how to do this and it feels good!) and if there's a chance that's the case, neutering could make the problem worse.