Call the river and make a note.
He could have a ten and be bluffing
a missed straight draw.
He could have AJ and be block betting.
He could a missed turn FD (floating a scary board with the intention of raising the turn .. but then calls as he picks up a FD)
He could have QQ and be block betting
Bet the turn bigger and cal this sized river bet then make notes.
You get to find out if he raises / calls his draws on the flop and or turn. You get to see if he randomly calls intending to bluff the turn.
You also get to note what he calls 3bets OOP with AK QQ JJ KQs etc arent all that interesting but 67s? Calls for a GS on the flop, picks up a FD or even calls twice with a GS and then bluffs river?
Maybe he has AA but then you can note that and play 3bet pots better with him.
There are enough missed draws and second best hands to call this river bet with.
Also note that he either bluffs rivers cheap or value bets rivers small. If you see him do this with AKs for the nut flush then his big river bets are more likely to be bluffs than small ones.
Maybe he slowplays a straight here, again note that he slow plays a straight on a FD board and then thin value bets it on the river when the flush comes in.
You say he is aggressive but he is calling down with a draw (if he has the FD) he is also likely calling a 3bet OOP with a weak hand. Those things arent all that aggressive.