I do agree we shouldn't mollycoddle them too much, however tempting - they need to learn the realities of life for their own good.
If at 23 he is living with you on a long term basis, instead of choosing to get his own flat, then I'd charge him each month. What you ask is very reasonable, it's his choice if he stays away a lot.
I too would be tempted to reduce it, but I think you should stand firm and ask for the full rent, for his own sake. That's what a landlord would do.
You could stash some of it away, unknown to him, then when he does move on you can give him a little gift to help.
I wouldn't like having to ask each month. I would have one conversation where you explain you don't like the current set up, insist that he sets up a direct debit now or gives you the money on pay day without you asking.
I must admit that when my adult son came back and stayed for some months, I didn't charge him.He didn't have a job, was in a bad way, needed parental help. It's different if it's a permanent arrangement and they have a job, in my opinion.