AA on a safe board, villain takes strange line

[ChoicestHops]

Full Tilt Poker
$1/$2 No Limit Hold'em Ring Game
6 Players
LegoPoker Hand Converter

<font color="black">Stack Sizes</font>
CO: $398.80
BTN: $103.60
SB: $325.45
BB: $225.8
UTG: $318.50
Hero (MP): $254.60

<font color="black">Preflop:</font> A[img]/images/graemlins/club.gif[/img] A[img]/images/graemlins/spade.gif[/img] ($3, 6 players)
UTG folds, <font color="red">Hero raises to $7</font>, CO folds, BTN folds, SB calls $6, BB folds

<font color="black">Flop:</font> 3[img]/images/graemlins/club.gif[/img] 2[img]/images/graemlins/diamond.gif[/img] 7[img]/images/graemlins/spade.gif[/img] ($16, 2 players)
SB checks, <font color="red">Hero bets $10</font>, SB calls $10

<font color="black">Turn:</font> 3[img]/images/graemlins/club.gif[/img] 2[img]/images/graemlins/diamond.gif[/img] 7[img]/images/graemlins/spade.gif[/img] [Q[img]/images/graemlins/diamond.gif[/img]] ($36, 2 players)
<font color="red">SB bets $20</font>, Hero calls $20

<font color="black">River:</font> 3[img]/images/graemlins/club.gif[/img] 2[img]/images/graemlins/diamond.gif[/img] 7[img]/images/graemlins/spade.gif[/img] Q[img]/images/graemlins/diamond.gif[/img] [6[img]/images/graemlins/club.gif[/img]] ($76, 2 players)
<font color="red">SB bets $100</font>

Villain is a 9/4 after 60 hands or so. Villain calls a flop bet 12.50%. He bets on the turn about 33%, and bets the river 25%. When he bets the turn, I felt like he has a set. I never put a player on a set as this is pretty stupid, but his line didnt make sense to me. He calls with any pair here preflop I think, and I cant see him cold calling OOP with a hand like AQ. But I dont think I can completely rule it out. KK is a possibility here as he is so nitty he may not re-raise it pre.

At the turn, I can either raise his bet about 3.5x, but my plan was to let him hang himself. But when he bets on the river, an overbet, and basically never bets the river, he either is putting on a bluff or he really is strong and wants me to call. I dont know if he sensed weakness from me or not, but I feel like the overbet wants a call. I could see him maybe doing this with a missed flush draw. What do you think and what would you do here?