[Acroyali]

I’d keep swimming him and ignore the odd comment suggesting people think you should starve him.
Swimming is low impact, which is excellent on oddly shaped joints, and muscle building.
Building muscle while removing fat is critical here. Swimming will help this along but his stamina will need built which can take a few months, but can certainly be done.
Swimming helps MUCH than walks for weight loss/muscle gain, and is way more fun for most water dogs.
(Don’t go by the scale alone. Take pictures and date them, and take pictures every 2-3 weeks if you have to. One of his entire body side view, and one viewed from above..his back/waist, etc. GOOD weight loss is slow and steady til a plateau hits. Bad weight loss is rapid and is usually water/muscle loss, not what you want with a dog with poor hips. Good muscle supports poor joints.
It may be so gradual that you won’t notice, but in 6 months you may see the original photos and be shocked.)

Most serious dog people who have performance dogs swim them for these reasons.
Think of human PT and physical rehab. Most start with swimming because of the low impact for injured/unwell joints and muscle/calorie burning properties, vs. taking a walk or 30 minutes on a treadmill.
Letting him self regulate as you’re doing IMO is one of the best things you can do. When he’s tired, let him quit, then see if he wants to go again in another hour or so. He’ll build endurance.