Like Phaset said, I was X once.

The only advice I can think of, largely to help him out, is to be honest. Tell him you're not interested in him (either 'in that way' or 'at all' whichever fits best to how you actually feel).

We don't pick up on a lot of the signals that NTs do, so ignoring him or making up excuses to not chat right now may not tell him you're not interested. Especially if he's never really had any sort of relationship of this nature in the past to draw insight from. I know from experience how easy it is to misunderstand things when people use the wrong words. An actual ex of mine told me they needed "some time for themselves to think about things" and because I have AS (which I didn't know at the time) I took that literally and didn't realise for more than six months that we'd actually broken up. The fact I was made to feel like an idiot hurt a lot more than it would have had they actually just told me they weren't interested any more in the first place.