Any logic behind this guy's set weakness?

[GoCubsGo]

PokerStars No-Limit Hold'em, $0.50 BB (9 handed) converter

SB ($50.45)
BB ($46.65)
UTG ($27.60)
UTG+1 ($22.15)
MP1 ($54.10)
Hero ($49.75)
MP3 ($5.25)
CO ($20)
Button ($85.45)

Preflop: Hero is MP2 with 4[img]/images/graemlins/spade.gif[/img], 4[img]/images/graemlins/diamond.gif[/img]. CO posts a blind of $0.50.
UTG calls $0.50, UTG+1 calls $0.50, <font color="#666666">1 fold</font>, Hero calls $0.50, <font color="#666666">1 fold</font>, CO (poster) checks, <font color="#666666">1 fold</font>, SB completes, BB checks.

Flop: ($3) 8[img]/images/graemlins/club.gif[/img], 4[img]/images/graemlins/heart.gif[/img], A[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(6 players)</font>
SB checks, <font color="#CC3333">BB bets $1</font>, UTG folds, UTG+1 calls $1, Hero calls $1, <font color="#CC3333">CO raises to $2</font>, SB folds, BB folds, UTG+1 calls $1, Hero calls $1.

Turn: ($10) T[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(3 players)</font>
UTG+1 checks, <font color="#CC3333">Hero bets $7</font>, CO folds, UTG+1 calls $7.

River: ($24) 9[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
UTG+1 checks, <font color="#CC3333">Hero bets $8</font>, UTG+1 calls $8.

Final Pot: $40

Results in white below: <font color="#FFFFFF">
UTG+1 has 8d 8s (three of a kind, eights).
Hero has 4s 4d (three of a kind, fours).
Outcome: UTG+1 wins $40. </font>

I was really surprised to see what he had here. There are very few hands that beat him, and based on my play, I can't possibly have them. I was going to the felt with this one thinking he was donking Ax or maybe he had 2pr. Is there any reason you can think of for him to check call this river or should I add him to my buddy list?