The gaslight theory is possible, but I didn't get that vibe. I really doubt the idea that he was just going home from work or a better restaurant, since it would be much easier for him to use the official transfer point between routes while carrying a large box horizontally at his hip.
Other theories:
He was dropped off by a friend giving him a ride halfway home. This is on shaky ground because it would have been more practical to drop him off further north, which would have been further along the bus route. It would make sense only if he was trying to catch either of two routes that stop there and have the same destination, but travel some distance apart for most of their runs. The destination is quite some distance away in Jamaica, Queens, with access to the subway system. If he was going to Jamaica or somewhere down the subway line, the pizza would be VERY cold by the time he got home.
He had business near where he got on. Again, this would be awkward carrying the box, and the pizza would be frozen at the end.