It's ok to talk about eating for health. The damaging thing is linking weight, overeating or eating treat foods to emotions like shame, greed, disgust.
But by not talking about his physical health you're doing him a disservice. You're prioritising his emotional health over his physical health, and realistically he knows he's overweight, he's going have feelings about that anyway.
You can talk about food as fuel, becoming stronger, becoming fitter, having more energy to live a full rounded life etc. Talk about food being useful... carbs/sugar =energy, protein=muscles, etc etc. Nothing is bad, it's all just about portion sizes. You can also talk about poor health outcomes for overweight people eg diabetes, heart disease, lack of energy, joint problems.
Practically, I'd only by treat foods that were individually portioned and not have many in the house. I'd make sure to provide him with more protein and more fibre in his home cooked meals to keep him full for longer. Does he eat breakfast? I'd make that high protein/high fibre. The longer he feels the satatied full feeling, the less likely he is to crave sweet stuff.
Has he had his bouts of diarrhoea looked into? Overeating as a cause of this wouldn't be at the top of my mind. An excess of sugar can alter your microbiome (especially if it had already taken a hit due to a course of antibiotics) which could lead to gut issues/IBS type symptoms. But I would want to exclude any other potential causes. The gp can arrange for a stool test to look at a range of markers.