From what I can gather, the actual amount he will cost is 50pw. So tell dh you don't fancy making a profit of your son and charge him 50pw.
You can't compare living in childhood room with parents to private renting. Even if you could, I currently pay 325pm rent. That's lodging in a family home (clearly I put my disposable income above having my own place...). Yes I do have to buy food, but if I was being frugal and living off pasta pesto I could easily live off 400pm all in. That's including bills. And living outside the family home.
The question isn't what son earns or how much disposable he has or how he earns more than parents. It is, or should be, how much he costs and charging him that. Comments such as save for him etc are patronising.
Higa who died and made you God? Why do you think you can dictate how much disposable income young adults 'should' have. I tell you now, when you discount food and bills, I have double if not triple that figure leftover each week. Unless you're on UC or have ridiculous bills it's not unusual. I then spend that on counselling, gym, Netflix, now TV, a good phone contract, nights out, nice food, a daily costs coffee etc.
Nothing wrong with that. It's my money, which I work hard for. If someone wants to bring a flask of coffee and pay a mortgage that's their business. I prefer to pay low rent and having less space and more Money.
Also I'd love to know what disastrous consequences you have in mind. Personally I'd think being short on cash or feeling deprived to be more risky. You know, temptation. It's surprising how many people I work with who commit crime to have a nice lifestyle