SAS says that he wants to stay with this boyfriend and to have sex (within his current relationship) with someone he finds more physically attractive, i.e., he wants this boyfriend to get in shape. SAS sounds like he is willing to put some effort into this, but he doesn't tell us how much effort his boyfriend wants to put into it. If the boyfriend is willing but not eager, than a good start would be for SAS to start asking his boyfriend to go for a 1/2 hour walk with him every evening after dinner. It is easy and enjoyable, even for the out of shape. Spending this time together could strengthen their bond. If walking goes well, then in a couple of months, the boyfriend might be interested in a more strenuous activity -- racquetball, jogging, swimming, etc..