Cats can still get erections after being neutered. They just can’t ejaculate. Some never show signs of sexual awareness, and some have a favorite blanket/towel/stuffed toy/human they like to hump on the regular. One of them lives with me. He likes a certain corner of the couch, and a fleece blanket that’s on the bed. Eh, as long as he leaves me alone, it’s no big deal. My other boy used to try to hump my arm, including pinching my skin in his teeth to try to scruff me. Ouch. No. He stopped that after he had PU surgery. Poor guy.
One small possibility is that when the cat was neutered, he was partially cryptorchid and only partially neutered. Sometimes the cryptorchidism corrects itself and the second testicle descends later and he’s a totally viable intact male. Or doesn’t descend but still works fine. Needs to be neutered again. Sometimes the second testicle is not viable and remains cryptorchid, only findable through exploratory surgery. I’ve seen them found in a cat’s thigh. Or, most rarely, a piece of testicle somehow didn’t get excised during the neuter. In those cases, the cat may develop all the secondary sex characteristics of an intact male (the huge cheeks, stinky urine, barbed penis, marking, trying really hard to get out of the house), as the testicle material may not be viable for him to reproduce, but emits testosterone for him to develop fully.
One of the first things a vet will check when a cat is known to have undergone a neuter procedure but is displaying sexual maturity, is to check his penis. If barbs are present, there’s testicle material present, even if it can’t be seen externally, and an exploratory is called for because there’s likely a viable testicle that never descended. If there aren’t any barbs, then what testicular material may still exist is likely not viable and at that point it’s kind of up to the owner whether they can live with the behaviors/smells, as it becomes more possible the doctor might not be able to find it during an exploratory, which is very invasive/expensive abdominal surgery.
Most likely in your case, flatlined, he’s just a regular old neutered male who likes tummy rubs.