FIRST check - and having found him have a look at his kill style & habits in Zkill, the kind of thing he likes to fit to his current ship where he hangs out who (so possible alts?) he kills in company with.
SECOND launch your mobile depot fit up the ship scanner you have in cargo lock him up & check out what his actual fitting is right now.
THIRD equipped with any details gleaned from the FIRST step about potential alts & allies check the local chat channel to see if he has any friends nearby… if he’s in a player corp set it to orange 1st so you can spot any corp mates easily, ditto for alliances.
FOURTH assuming local is clear of any of his friends refit (in space from your mobile depot) a hard counter to his current fit & go for it.
If you need to keep them on grid long enough to do all that then talk to them, @Buoytender_Bob’s aproach is perfect for that.