100NL - Weird spot with a flopped set against a reg.

[thing85]

Villain is a pretty solid reg, 21/18/2.5 over large sample.

Poker Stars, $0.50/$1 NL Hold'em Cash Game, 6 Players
LeggoPoker.com - Hand History Converter

UTG: $83.20
MP: $83.65
CO: $44.50
BTN: $266.05
SB: $165.80
Hero (BB): $107.35

Pre-Flop: 4[img]/images/graemlins/heart.gif[/img] 4[img]/images/graemlins/spade.gif[/img] dealt to Hero (BB)
4 folds, SB calls $0.50, <font color="red">Hero raises to $3</font>, SB calls $2

Flop: ($6) A[img]/images/graemlins/diamond.gif[/img] 4[img]/images/graemlins/diamond.gif[/img] J[img]/images/graemlins/heart.gif[/img] (2 Players)
SB checks, <font color="red">Hero bets $4</font>, SB calls $4

Turn: ($14) 5[img]/images/graemlins/spade.gif[/img] (2 Players)
SB checks, <font color="red">Hero bets $8</font>, SB calls $8

River: ($30) K[img]/images/graemlins/club.gif[/img] (2 Players)
SB checks, Hero...?

Villain is solid and probably knows I'm solid. It's completely strange for this type of player to limp in the SB. IMO, villain probably raises any of his BTN opening range. Is he getting fancy with AA/KK? He knows I'm a thinking player. My bet sizes are probably smaller than most people would advocate, but they're enough to keep him from drawing to a flush profitably.

Is he calling me down with air hoping to bluff a 3rd diamond? Or is this a poorly played AA/KK? Certainly he'd throw in a raise somewhere with AJ/AK/KJ/Axs/etc (especially PF). There's probably no way I get away from this hand even if I do put him on AA or KK, but I'm curious to see what others think.