Veterinaria La Hacienda

This is going to sound weird. I have an 8 month old female cat who keeps “sucking” on a 4 year old neutered MALE cat. The older cat seems as though he is in pain after the “nursing” has subsided. I’m concerned that this may also mean that the younger cat has severe mental problems. I’m assuming that this behavior is an effect of the cat not being properly weaned. What can I do, or does anyone suggest something that can be put on the older cat’s nipples to stop this from recurring?