You need to ask yourself two questions and the answers should help make your decision pretty easy.
First, which matters more to you: the quality of your relationship with this person, or all the other things in your life that you listed.
Second, are you comfortable being in a relationship where your partner is likely to find other women more attractive for something you yourself could change but do not wish to?
Most likely you will gain with him, even if not intentionally. Chances are, your excess weight will last longer than the relationship. So, if you are happy being fat, stay with him. If not, find someone who appreciates your current size.
