I think the withdrawing thing is very specific to the individual Vixen and I can only speak from my perspective...
I am, if I'm honest with myself, somewhat afraid of getting too close to a woman, at least on the emotional level. For me, that largely comes from the thing with Miss I that some of you will remember at the beginning of the year - I've never been like that before, it was scarily out of character, it messed with my head to a degree and I don't want to go down that road again.
Thus, if I were to ever feel that pull again, I would start questioning my feelings but, more so, my judgement. For that reason I would want to back off and process, possibly even end things if it felt too uncomfortable.
If it was me, and I don't consider myself to be needy in any way, shape or form, I would probably want some reassurance that it is ok to feel what I feel but, more than that, the space is important.
I suspect us men process emotional stuff in a very different way to most women, i.e. I am very clear on what I feel but there is no way in hell I could verbalise that to another person without getting seriously tongue-tied and confused in how I am saying it.
I'd suggest you do message him but keep it light and let him dictate the frequency for a while. If you don't give him some space he may well withdraw further.
Hope that makes sense and hope it helps!