Is it possible some kind of bad fortunate has befallen this person, and they are unable to reply to you?
Or do you know for certain they are in good health, and just aren't responding?
If it's an issue of him just not replying back for unknown reasons and you are romantically interested... I'd say knock once, wait a bit, knock twice, wait a bit, and then knock a third and final time after a while; and if there is no reply, you'll just have to close that chapter. Maybe wait it out a few months or so.
There's not a lot of information for me to really make an informed decision.
I think the important question to ask yourself is. How long would I be willing to wait for a reply to come back? Would I be resentful if I waited a long time and finally did get a reply? Am I resentful right now?
It sounds like you'd be willing to sacrifice what was forming, if you could just go back to being friends, and I don't know for certain; but, I don't think that's the best way to think about things. I think, that I'd want my partner to BE my best friend, and there is always risk involved in stuff like that.
Anyway, I think that's the best I can come up with. A good friend, to me, is worth waiting for; but, nearing my 40's now, it seems, even long-standing friendships can fall out of favor, unexpectedly...
Love is all about timing, I think... Sometimes the timing works out, sometimes not..