The warlock bit is tricky though, fine line between a kid simply making a mistake out of ignorance, with a new born talent and one who has made a habit of it. The former can be redeemed the later cannot and the Merlin was right about them.
According to Zoo Day, Mouse seems to be able to do something about the warlock taint.
Harry would still have to be selective, because 1) the Council doesn't know that Mouse can do that and 2) cleaning off the addiction to doing bad things is only helpful insofar as the warlock being rehabilitated doesn't actually want to keep doing more of them.
As for involving Tilly, that's a double-edged issue. If they get him too close, the Librarians are going to assume he's compromised by the supes (WOJ is that they never reached out to Murphy because they thought she was too close).
Tilly is someone they'd probably eventually contact with questions, so long as they don't think he's going to run back to tell Harry about it. Harry's best move is to mostly leave him at arm's length so his account that Harry seems OK in his limited experience will be treated as somewhat objective.
Ideally, pay Tilly a courtesy call to inform him he's back in town after having to lay low, but don't get too close. He's more useful as a potential intermediary to eventually open diplomatic contact with the other Feds than to recruit as an immediate ally.
