It may very well be a dominance issue, but don't rule out the hormones entirely. Neutering won't necessarily stop it in either case. I've seen many altered dogs with the same habit. You'll have a better chance of it working, though, if you do it before he hits puberty.
I'd really encourage you to switch him to an organic food, not just because of the humping habit, but for his general health. Different dogs do have more pronounced responses to hormones, though. Our Buffy kept cycling into season at odd times, even in her too-short life, and we finally realized that it was because she was finding the afterbirth when one of the cows calved and eating it. The hormones in it were kicking in her cycle. Shiva's found and eaten afterbirth (ugh - you try not to get kissed for awhile afterward!) but it doesn't seem to phase her; possibly because I finally convinced Charlie that chemical and hormone laden commercial food wasn't good enough for our dogs so she's not getting the extra hormones regularly from her food. I know, though, that finding a readily available organic food where you are may not be feasible. Where I live in East Tennessee, it's actually easier to get wholesome, organic food for my dogs than it is for Charlie and myself! I'll have to admit, sometimes the ginger cookies I get for them start lookiing good with an afternoon cup of tea. So far I haven't given in to the curiousity, though! (lol)
How big is HUGE? Since he's a Cocker Spaniel, he's still small enough for you to be able to use very moderate aversion-type discipline. He might respond to being squirted with water, or, if that doesn't phase him, try something that makes a very loud noise, like a balloon popping.
Keep us posted!