You either accept his invitation, or you turn him down gently. Either way, I think you should thank him for asking you.
I believe it is appropriate to set a few guidelines when accepting a date so there’s no room for confusion later. For instance:
‘Thanks, that would be great. But you need to know I’m not interested in any kind of physical relationship right now, so I understand if you want to change your mind. But if you’re happy to go and hang out and talk and get to know each other better, I’m up for that."
If he tries to cross the non-physical line, you should confidently remind him of your agreement. But if you do want to get physical, like kissing, you’ll need to revise your rules. Otherwise, you might get stuck in the friend zone forever.