125 lbs. @ 8 years old? That’s very, very heavy. I’d be surprised if my 8 y.o. weighs 60 lbs. So I can see where it would be a challenge to get him engaged in team sports at this point. Here’s a point of reference.
1. Talk to your son about his weight. Ask what he wants for himself. Without his buy-in and cooperation, you will find progress difficult… especially with separated parents.
2. Keep on talking to him about it. Not judgmentally… but with open questions. Try focusing on what he would like for you to do to help him.
3. Continue talking to him about it. Recognize and reward his progress frequently. Tell him you’re proud of him and how much you love him.
4. Do recreational exercise together… find out what he likes and stick with that. Biking, scooters, playing at the park, frisbee, toss a football, whatever. Take him to professional sporting events to pique his interest, like a hockey game or a baseball game. Don’t make it about losing weight… it should be fun.
5. Some amount of sensible weight training might be fun for him… especially if he’s stronger already than his peers.