Ha. Ha ha ha.
Recognised myself in your DP. My nickname is very out of date.
I've always been broad-shouldered and powerful, and enjoyed endurance sports - marathons, triathlons, long-distance cycling. I was VERY fit, and needed to eat about 5000cal per day to feed my training.
Then life changed, but my eating didn't. I put on 2kg a year for the last five years, and 6 inches around my waist. I knew I was getting heavier, but could still cycle to work 20 miles a day, go dancing, walk for hours.
Buying new trousers in january forced me to confront the weight gain. 40'' waist, up from 34'' 6 years ago. 98kg, up from 88 6 years ago. Nightmare. I bought the GI diet book, a weighing scales, a tape measure, and started watching what I eat. I also asked DW to take photos of me with date, weight and girth.
I'm down to 92kg and 38'' now, and feel much better, both in myself and about myself. I'd like to lose another 4kg.
All of which is a roundabout way of saying that your DP will do something about it when he can no longer deny to himself that it's happening. Perhaps you can help that to come about by putting some "before" and "after" photos together, or digging out a pair of his old trousers, or something. The trouble is that this is confrontational and may entrench his position more.
Has Steve Redgrave or anyone done anything on weight loss for rowers? Or the Oxbridge boys? I know that they go through a long process of "de-training" after they stop rowing, to ensure the muscle doesn't turn to fat and that they eat properly. Sometimes a message is more easily received if it doesn't come from a loved one.
I hope you get it sorted.