According to the books, Wesley never gave up or lost his commission, so, as he wanted to attend his chums wedding, and he was still entitled to wear a uniform, it was a simple choice for a formal occasion. He went back to travelling after the wedding.
I have dug out the book and scanned it, and in the last book of the A Time For series, A Time for War, a Time for Peace, page 305, about halfway down, Wesley appears.
Expecting a Betazoid wedding, he is naked. It must be fair to assume that Picard would know if Wes was still entitled to wear uniform, whether he was retired, inactive but still on the roll, or whatever. He calls the ship for a uniform, as Wes's size, etc is in the computer.
The Q.M. has told Mr. Wreide, the officer that answered Picard's call, that he has only an L/t uniform and the replicators are off line for servicing. (You know, just like warp drive goes off line when the writer wants a ship to stay put). I cannot understand why a clip on pip cannot be changed, but I am guessing that, the continuity department cocked up on the film, and this is cont/dept excuse number 7.3.
So, if nothing else, it was just a small detail that slipped through the cracks, like a Roman soldier wearing a Rolex. I try not to let these small details ruin my enjoyment of a good story, but hey, if that's what you like, go for it. I hope my time was appreciated.