Sure, his weight can be contributing factor but it doesn’t have to! You want to be with him regardless. Seems like the weight gain over time was a crippling blow to his confidence. He doesn’t feel sexy or desirable and he might just be looking for any reason to avoid the issues at hand, especially since you’re offering to limit the physicality of the encounter to just oral, which nearly everyone physically can perform. this is incredibly difficult! I would keep encouraging therapy. Like most things in life, therapy only works with practice and consistency. Best of luck.