According to the circuмstance you describe and the amount you proposed, rent seems fair. If you need the money, he should in charity, carry more of the families burden. If he is planning his own family then he should be encouraged to save up. My son is 25, lives at home, has a full time job and pays rent. There is food, heat, wash, water, and all sort of things to pay for and a grown man should pay his way.
God bless, JoeZ