Just make sure it's a good franchise he gets to... and not something he thinks Dean will be proud of him getting into. You know... along the lines of "The Pizza Man"... probably not the best franchise to be fangirling over, lol.
He needs to be a fan of the Mortal Instruments. It's right up his ally. Angels, demons, and other supernatural creatures. It'll be familiar to him and yet I'm sure he'll learn quite a bit about what it means to be human from it.