Apologies if I sounded snarky. Sugar addiction is a difficult problem. If they're on board to any degree that's already a lot of progress. I'll pray for your family.

A plan will help, if you don't have a thorough plan already. There's a danger of subbing carbs (starches like chips or cereal or mashed potatos, breads or even alcohol) for sugar. Carbs are stealth sugars, causing the same long-term issues, but seen as harmless by many.

I recommend the carnivore diet, coupled with a high intake of sea salt. (The reasoning behind it is here: https://darkovelcek.wordpress.com/introduction/ and it's worth the time to understand any plan before setting out. At least for me it makes compliance much easier. He has a lot to say about inflammation and cancer as well.) After the initial change period, it's invigorating. I can feel my health improving almost daily, very easy to feel over the first few months.

It's a big shift, but worth it, if you can get the buy-in. Might be easier to start just by going low carb. I did that for a few years (and it helped- lost weight gradually and painlessly, improved circulation and energy), just a big reduction of dietary starches, before I learned about carnivore and decided to do it. Your son can recover.