Here's my thoughts. I have over 13 years in the field.
He's disabled and only gets a smallish check each month. I think you need to check the legality of what you're thinking about doing. If you are his payee it could be seen as you keeping his money away from him. It could also be seen as too much if you kept that much.
I understand you could really use the money to help out. I also know how hard it is to manage things with a situation like this.
He is already helping and babysitting. Do you pay him to babysit and do chores? I imagine not. In a group home he would not have chores to do that were required. He would have some chores but if he choose to not do them he would not get into trouble. Housing is not dependent on whether he does this or not. It's a thing they do to be helpful and help their neighbors have a healthier living area.
In a group home he would have goals such as learning about money, managing his money. Of course completely doing that is a long long long long way down the road and probably not something one would be able to do wonderfully. He'd have friends and staff around all the time. They'd go to the movies, out to eat at least once per week, go to the grocery store, learn about shopping and finding their way around inside if they got lost from the group.
They'd have goals to learn how to do their own laundry, wash their own bodies, brush their teeth, shave, get dressed each day in clean clothes, they'd have a lot of things they'd be working on each and every day.
So if you're not providing these things for him are you just letting him sit around all day and not improving his skills? What does he do all day?
In a sheltered workshop he'd participate in filling contracts, he'd eat his lunch with his friends and do activities after hours, he'd go to dances and parties, they would help him learn skills to move up at his job to something better.
He'd be doing all this and living on his own and paying rent. They'd be providing staff, housing, food, medical assistance for his meds, they would be doing a lot for him all the time.
I have a niece that moved from her mom's home into a shared living situation. She held a full time job, was able to get a taxi to and from work when she needed a ride, go to the store and get her own groceries (it was across the street), and she was able to live without staff.
Her mom came by one day and caught the daughter and the boyfriend having sex. Come to find out the daughter and the boyfriend had been having a sexual relationship for years.
This woman was in her late 20's or early 30's. She had an IQ in the high high high 60's. 70 is the line between low IQ and low normal IQ. So if she had one or two more points on her IQ she wouldn't qualify for any SSDI or other disability income.
There is a man that lives in one of the group homes I was manager of. He works full time in a grocery store, he walks to work, gets himself home, stocks the shelves, run an occasional register, and is a right hand to the owners. His IQ is very similar to my nieces.
His mother is very aged. She takes his entire check from DDSI and his pay check and cashes them. She gives him a $10 allowance each week. She keeps the rest. She does pay his $300 month rent at the home. She keeps the rest of his money and no one can get her to say what she does with it.
She'll only say "He doesn't need it, he's retarded you know". That's a situation that isn't good, this man supports himself and is quite able to manage money and do anything he needs to do.
He won't have any freedom until his mother passes away. He won't have a penny of money she has I bet but once she's gone he'll be able to have his money in his own checking account and won't have anyone to tell him what he can and cannot do. If he wants to have a payee or a team to help him decide what to do he can ask for help. Many do because they don't want their family to take all their stuff.
So what I'm saying is to please consider a group home, shared living, supported living, well supported living might only be in Oklahoma, and try to make it where he can learn skills today that will help him in the future. One day you won't be there anymore and he'll be in a group home or shared living home then and won't have you to do stuff for him.
Let him attend a sheltered workshop so he can have friends and be out of the house all day working.
Help him to gain independence and live life to the fullest.
Should you charge him rent? Sure. Should it be $200 per month? If his check is only $250 per month then no, that's too much and you should be held accountable for what happened to him income.
If he makes $2000 per month then $200 is a good percent of that income to pay for rent for a developmentally disabled person where no professional training is occurring.
Should he be helping out during the week? It sounds like he's doing that already.