I have not regretted having either of my boys neutered. If your dog is gaining weight it is because he's eating too much and not getting enough exercise. Bogey was neutered about 2 months after I adopted him and he was a little over a year old. He was skin and bone and boy did he gain weight. He was 5.2 lbs of nothing and he gained and gained and gained............all the way up to 14.5 lbs!
The issue with him was he was extremely underweight and around a year old. The vet told me he needed to gain but the problem was I forgot to stop. He's now down to 10.4 lbs and the vet said he is perfect. But I never blamed his neuter on his weight gain. I took full responsibility and changed his food and cut the amount back. He's very friendly, loving and he runs and plays all the time.