Definitely get him checked. There are a couple of possibilities. If he was cryptorchid when neutered the first time and the vet for some reason was in such a hurry or something that he didn’t notice he only removed one testicle, the cat could have descended the other testicle in the meantime. You could do your own physical check - if it feels like he’s got a good-sized grape near his penis, then his second testicle descended. Or it never did, and he’s still got a retained testicle and actually needs an exploratory surgery to find it and remove it from his lower abdomen. The vet you take him to will check that the cat’s penis is smooth - unneutered cats have small barbs on their penis. If he has barbs and no visible testicles, then he’s either cryptorchid (sometimes the other testicle can be felt during a physical abdominal exam, or a super fancy vet might use ultrasound). Or - it’s more rare but possible that if the vet who did the neuter didn’t get all of the testicular material, and if there’s enough left it can continue to release enough hormone to make him develop all the secondary sexual characteristics of an unneutered male though he still can’t actually reproduce.
As to the humpy thing - some cats just still do that whether they’re neutered or not. I’ve got one who will get all comfy on the couch and then start trilling at me and biting my arm and then starts trying to get busy. Gross, and he gets thrown off the couch for it. Another of my boys only does it to the other cats and it’s never tolerated - usually turns into a “play” fight that stays friendly but clear to knock it off.