Is he round? Round is a shape.
Do YOU work out? Invite him to join you on a run or something. Make it a perk and a bonding experience that you share, not an obligation for him.
Aren't you just after the body then? The fit kind of body? Do you love him as he is or you love him only when he gets skinny/fit?
Leave him alone. Fitness is a PERSONAL journey.
hold out on sex
lazy or does he smoke weed
Nag him until he leaves?