Careful with that advice, I have had and heard of a number of cats who have sprayed well before 6 months, oh even my little female kitty. I didn't know at the time and I waited until 5 months to spay her and she sprayed all over the place hehe. I wouldn't just count on the vets advice. Kittens can be spayed and neutered at 8 weeks old, many vets do it, I would find another because waiting until 6 months is just asking for trouble
Especially since you had trouble in the past.
Another small red flag from that vet is that he recommended Iams, unfortuantely many vets do because guess what they usually sell? Iams! hehe. It is not huge deal, especially since that is not what the thread is about. I tend to get off topic sort of. But just know that Iams is not a very good brand of food, maybe do a little research on here for some better options. Vets are not nutritionists and often just recommend what they sell.
Anywhoo... make sure you have at LEAST 2 litterboxes, he gets neutered ASAP, and maybe confine him to a small room until he is neutered so you don't take any chances. Like a bedroom, laundry room, breezeway, or even a finished basement.