But you have to sit out for a year if you transfer, so he wasn't eligible to play until 12 months after his transfer.

He arrived at BYU on Dec. 13, 2014, meaning that he is good to go on Dec. 13, 2015. Since the bowl game is after that, he can play in it.

His case is unusual because normally players transfer after the season is over, not during it. In that case there is no ambiguity because an entire football season occurs during the 12 months that they are sitting out.

So in summary: last year was his redshirt year, this is his FR year, and next year is his SO year. He sat out for 12 months which spanned part of his RS year and most of his FR year. After this year he will have 3 years to play 3.

Hopefully that makes sense.