obviously he should pay something.
but you shouldn't be 'charging him rent' either.
if your a healthy/mature couple, then you just work together. A big part of that is working together financially - making sure that you share costs, both get nice things, both take responsibility for jobs that need doing etc.
If my wife had asked me to pay rent back when she was my girlfriend and I stayed with her - I would have been quite offended. But we both had the mutal expectation that we would help each other out financially, and we both did. After a while, when we were fully commited to each other, we just pooled our money together, and started to treat us as a single financial unit, rather than two individuals - after that everything became a lot easier.