Cats can be neutered any time after 8 weeks, and I would reccomend having it done before 6 months. You can call your vet anytime to book an appointment. I would ask him/her what (s)he thinks!
I called my clinic today, and the response I got from them was about 4 1/2 months. Dinah is spayed, so I dont have to worry about her having kittens. My worry is him getting other cats pregnant. I waited untill after I had Dinah spayed to let her outside. Should I do the same with ScatCat?
Also... how much different is neutering than spaying? How much healing time will he need?
Probably best to keep him indoors and away from other cats until he is neutered. In fact, it is probably best to keep both your cats indoors ... the dangers of the great outdoors are too numerous and unless you can provide them with a safe enclosure, then it is much better to keep them out of harm's way.
As for the difference in neutering and spaying, the techie term for neutering is actually Feline Castration - and it is exactly that. They make a tiny little incision and remove the testicles. The healing time is minimal, usually a few days to a week to remove any stitches they may have placed. It takes about a month for the "plumbing" to completely stop being functional, so even after he is neutered, keep him away from girls who may not be spayed - he can impregnant them for several weeks after his procedure.
For females, the techie term for spaying is actually feline ovariohysterectomy - they remove both the ovaries and the uterus. The incision is a little larger and it is located on the lower belly. The healing time is again about a week before the stitches (if they used them) are removed.