This is a value shove from a boat or an 8, maybe even aces /kings if he's a real fish. No one at 5nl is good enough to bluff shove over your big river bet and its not a bluff that works here anyway because nearly all players in our position here are not good enough to lay down a strong non nut hand in any case.
I would need to know his stats before deciding whether to call. fish = call. tight player = Fold. Low aggression= fold. reg with high agg 50/50. You have to call sometimes because he is unlikely to put you on a straight.
18 combos of sets 666 222 TTT , 1of quads, 8 of A8, maybe 12 of AA/KK . Maybe its just a fold especially as you haven't invested much in the hand. Im pretty confident most 5nl are calling here though.