I understand your concern about neutering your cat now. As cats get older, it gets riskier. This is because of the anasethsia. However, ten years old is not old enough yet to worry about that. You can speak to your vet about your concerns, but he will prob tell you not to worry.
That said, I agree with patriciascloset that if he suddenly started spaying after 3 years it may be some other medical problem. (Kidney problem maybe?). You should call your vet and speak about possible causes and solutions. If it is just spaying, definitely get your cat neutered! There are so many health benefits to neutering a cat or dog. They will even live longer!
Good luck!