Small blind is a passive donk (started halfstacking) and we have some history. I 3bet him first orbit with AKo, he called, and we checked down a Q10xxx board (he had AKo). Another hand I 3bet with AA he calls and he checkraises allin a Ks10sx flop with AKo (he won). I should get a lot of respect from my 3bets. He called preflop instantly and bet the turn within 3 seconds. What is the river plan? He has $300 left.

PokerStars No-Limit Hold'em, $4 BB (5 handed) Poker-Stars Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

Hero ($683)
UTG ($912.40)
MP ($410)
Button ($551.85)
SB ($402.50)

Preflop: Hero is BB with J, 4.
1 fold, MP raises to $12, 1 fold, SB calls $10, Hero raises to $56, MP folds, SB calls $44.

Flop: ($124) J, 3, 9 (2 players)
SB checks, Hero checks.

Turn: ($124) 5 (2 players)
SB bets $40, Hero calls $40.

River: ($204) 7 (2 players)
SB checks, Hero?