I'm pretty sure they aren't casting him to play a young Han Solo. I'd imagine his character is supposed to be older. Which is why somebody like Chris Pine wouldn't be playing him.
Rumor is they're also trying to get Mark Hammill and Carrie Fischer to return to their respective roles as Luke and Leaia as well. But it's well known in the convention circuit that Mark Hammill is a money grubber. He'll try to milk the role for all it's worth. Last comic convention I went to, he was there, and was asking $100 a pop for his autograph (I sure as hell didn't pay him, so no autograph for me), when everyone else was doing autographs for free.