This idea that overweight people just don't know or understand and need a talking to is all kinds of wrong, and actually quite patronising
being overweight does not mean you are an idiot, or don't understand the health implications, or even know what you are supposed to do to lose weight (even though losing weight is more complicated than just eat less, move more, despite what a lot of people think!)
it's often about motivation, and somebody telling you "oh, did you know you have put on too much weight?", isn't going to work in most cases, as a PP said, sometimes even an actual health scare is not sufficient
It is a very complicated issue, often to do with mental and emotional well-being, which is not going to be helped by having a humiliating and mortifying conversation with a relative or parent, no matter how well meaning, in fact, if anything, this will push him away and make him less likely to seek your help on this and may even exacerbate any underlying issue
I think you'd be better off asking how he is in general and reassure him you are there for support no matter what happens in his life, which sounds like you would be as you are obviously loving and concerned