Villain is likely a fish, he had open limped CO once.
Is it fine to call river re-raise here? I think he could do this with AT or KT.
At worst case he has 35 combos for value (TT T7-T8 T5s 77-88 55 and J9). So he would need to have 11 worse combos. We will chop some of the time with 96. If he does this with AT and KT we have a call, or if he possible overplays worse trips.

partypoker - $1 NL (6 max) - Holdem - 5 players
Hand converted by PokerTracker 4

Hero (CO): $100.00
BTN: $100.00 (VPIP: 22.95, PFR: 17.59, 3Bet Preflop: 3.21, Hands: 606)
SB: $107.32 (VPIP: 27.06, PFR: 19.04, 3Bet Preflop: 7.07, Hands: 445)
BB: $86.84 (VPIP: 37.50, PFR: 25.00, 3Bet Preflop: 0.00, Hands: 8)
UTG: $141.16 (VPIP: 23.33, PFR: 17.78, 3Bet Preflop: 9.86, Hands: 184)

SB posts SB $0.50, BB posts BB $1.00

Pre Flop: (pot: $1.50) Hero has 6 9

fold, Hero raises to $2.00, fold, fold, BB calls $1.00

Flop: ($4.50, 2 players) T 8 5
BB checks, Hero bets $2.85, BB calls $2.85

Turn: ($10.20, 2 players) T
BB checks, Hero checks

River: ($10.20, 2 players) 7
BB bets $7.26, Hero raises to $28.00, BB raises to $81.99 and is all-in, Hero calls $53.99

Impossible to determine without reads. You say he probably is fishy but you have only 8 hands on him. If he is fishy enough to ship any T on this spot it is a clear call but players that bad are scarce these days and you can´t have that read with only 8 hands. Also J9s fits his shoving range perfectly as well.
I admit that this is the type of hand I struggle with on which I know I should fold but always end up calling

tbh I'd fold

Started to think the same once I realized J9 beats us. So is J9 the worst hand to call here?

Yeah.

I would find a fold. Not giving the villain any credit for a bluff here (his range is prob 100% value when he re-raises river). I think when villains range has no bluffs you can make some pretty epic folds and this is one of them. TBH i might even fold J9 against an unknown non-fish at 100NL

Raise smaller on the river, it's not like you have the nuts.

If you think there's a decent chance he's a fish, then I think this is a snap call tbh.