Firstly, neutering does not cause weight gain. It's up to you to keep your dog in good shape. Every pet has the potential to become overweight, and some are more prone than others.
Secondly, and most importantly, your dog will be many many times more likely to develop testicular cancer if he is not neutered. You would be doing your dog a service to neuter him. There are several good sites on the internet devoted to this topic.
I believe your boyfriend needs to do his research rather than making a decision based on emotions and lack of info. Just my opinion. Good luck!
