At age 19 our son obtained employment and immediately offered to take over the responsibility of paying a bill for us...cable and internet connection which with all the bells and whistles was at the time $111.00 a month. We have not charged any of our children board because quite frankly neither my parents nor my husband's expected this from us and we've carried that through to our own chiildren.
Our son was living on his own from age 22 to 24 but asked if he could stay with us to save money for his marriage which is the end of this month. In return he took over the cell, cable, internet and phone bill, without hesitation and without our expecting it. He has carried that bill on his own for 2 years. He does have it easier than most however as he works for my husband who has a company and thus he's never had to 'hunt' for employment and he gets a decent salary.
The amount your son is being asked to pay is absolutely miniscule compared to what he will pay in the big bad world of reality. If he does absolutely nothing except enjoy himself, perhaps it's time you gave him an ultimatum. Help or be on his own because quite frankly, $50/week is an incredible bargain and it's a shame he doesn't realize it. He probably eats more than that in fast foods in a week!
You'd be best to inform him that as of, say, April, you expect him to have a place of his own. That you've not asked for much from him, that $50 does not cover a great deal including any meals he eats at home or water he uses for showers, or laundry etc. If you are still doing his laundry? Stop! Still cooking and serving his meals? Stop! If the car he drives happens to be owned by you or you purchased it for him? Curtail his use of it.
This will of course cause all kinds of fall out, but you have raised this young man for 22 years now...he has it very good and he knows it. He has failed however to learn/understand the full extent of responsibility that a man his age should have acquired along the way and thus unless he obtains those lessons, he'll be a lost and incapable person next year, 5 years from now etc.
Stick to your guns, remain the parental authority (he's under your roof he follows your rules...doesn't like? Doesn't need to stay). If you hold tight, one of two things will happen...he'll finally agree, he'll storm out (probably sponge of Grandma), or he'll find he has to flounder in the big world and will appreciate what he had.
Good luck with this, it's tough being a parent, but it's tougher being one to an adult child who just doesn't want to get his act together and doesn't appreciate the breaks you have been giving him.