50nl : JJ overpair vs weird line from villain.

villain is 31/18 6.3% 3bet over 39 hands

Poker Stars, $0.25/$0.50 No Limit Hold'em Cash, 6 Players
Poker Tools Powered By Holdem Manager - The Ultimate Poker Software Suite.

SB: $52.71 (105.4 bb)
BB: $49.62 (99.2 bb)
UTG: $37.28 (74.6 bb)
MP: $54.71 (109.4 bb)
Hero (CO): $61.20 (122.4 bb)
BTN: $95.06 (190.1 bb)

Preflop: Hero is CO with J:spade: J:heart:
UTG calls $0.50, MP folds, Hero raises to $1.50, 2 folds, BB calls $1, UTG folds

Flop: ($3.75) 5:spade: 9:diamond: 4:heart: (2 players)
BB bets $1.50, Hero raises to $5, BB calls $3.50

Turn: ($13.75) 9:spade: (2 players)
BB bets $1.50, Hero raises to $8, BB calls $6.50

River: ($29.75) 7:spade: (2 players)
BB bets $1.50, Hero ???

think i should have raised his donk bigger on the flop. But got thrown by him calling my raise and then him donking turn small and thought he could possibly be trapping me. when he called the turn raise and donked again ,wasn't really sure where i stood as flush had got there , board was paired so he could possibly have trips or boats.It seemed he was inviting another raise but why call the previous two raises without a hand.

At this point I'd just flat his lame donk.