Neutered cats and dogs can still ejaculate. Personally I would not Vet but that is always the safe way to go. I would provide him with a washable toy or pillow and encourage him to use that instead of your arm. Our once feral boy was neutered at just past one year old and he still carries on with the feral female outside, just as you describe and the link describes.
https://www.drjeffnichol.com/masturb...ered-male-cat/