My vet wouldn't neuter Chewy until he was 2lbs. He's a lionhead and at 4 months he was only 1.7 lbs so I had to wait until he was 6 months. By then he was 2.4lbs and the vet was comfortable he could handle the anesthesia. I'm sure each vet is different, it was just what she was comfortable with. She did my sons lionhead and he came through beautifully, so I trusted her judgement. I'd say just take him in for a "wellness check" and see what the vet recommends.