Could this not also mean "What suit is he wearing"?
Why welk not welke
"Welk" is used for het-words (like "het pak"), whereas "welke" is for de-words and plurals.
Would "which suit should he wear?" Work too?
No, because then there should be 'zou' in the sentence
What would the difference between "does" & lets say, "will"?
I would guess that because "will" is in future tense, different rules apply as they do in english