Picard made him an Acting Ensign, which I guess made some sense. He wasn't an Academy graduate so therefore not an official officer. He was basically a civilian who was given a commission based on his skills and knowledge with pending entrance into the Academy. Learning on board the Enterprise earned him credits for the Academy.
Later in Season 3, after not going to the Academy two or three times for various reasons, Picard promotes him to a full Ensign. He still hasn't graduated the Academy so how could he earn a rank like that? Doesn't this suggest that anyone could continually be promoted on a Starship without ever having attended Starfleet Academy? If so, why didn't Harry Kim ever get promoted?
Eventually when Wesley does join the Academy, why didn't he retain his rank? He was essentially demoted to the rank of Cadet, correct? Was his Ensign rank put on hold pending his graduation?
I know I'm thinking way too much into this, but for some reason I find this very interesting all of a sudden haha