How to gently and nicely say this...?

He says he is uncomfortable with his body and he feels gross. He is not detrementally over weight by any means but he has a cute tummy on him. He's worked out in the past and lost the weight, I've noticed, but then he quits and it comes back. He says he feels pudgy and (I hate this word but) fat, but fails to do anything about it. I run XC and am I very good shape and I have several times invited him to go on short jogs with me sometime, or to go swimming, or to the lake, but he always declines saying "it's not his thing." Other than telling him I think he's perfect, I know losing the weight would make him feel more confortable in his body and skin, so how do I tell him he needs to get up and do something about it? WITHOUT upsetting him or making him feel like I think he should lose weight for selfish purposes?
How to gently and nicely say this...?
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