Why is it 'vado verso l'uomo' but 'vado verso di lui'?
I think because there is a difference between a "him" and a "man" - I'm not sure I got it straight enouh - But I think that has something to do with who are you talking about..
Is 'ad' not valid here?
Could you please write the whole sentence? I don't get what you want to substitute with "ad".
It's probably a mistake, but maybe not! :)
"Io vado verso l'uomo" > "Io vado ad l'uomo". Is the second one correct?
No. Not even "Io vado all'uomo". It doesn't mean anything in italian.
Io vado verso l'uomo, Io vado dall'uomo, Io vado in direzione dell'uomo
mean all more or less the same thing, but "verso" is the best translation for toward.