One idea is if you don't care about trophies, you could just play the digital games on his account for now, and use your current account for ps3. Later, when you are done with the digital games you can reformat the memory card and play the physical games, buy new games, etc.
It's still a good deal with the physical games.
If it were me, I would offer the $300 like you mentioned and pass on the digital games and tell him he can keep the log in to psn. If he ever decides to get a ps3 or ps4 in the future, he might be able to play some of those games.