unraised pot, strange situation

[G_Dollaz]

2/4 nl stars. Shortstack fish limps utg, solid 24/20 reg limps behind on the button (I've never seen him do this before but shortstack only had like 10bb or so). I check A3o in bb, effective stacks of $650

flop is 245 two tone, I bet $8, reg makes it $30, I make it $130, he calls.

Turn is the 2 completing the FD (aka the worst card in the deck). I check with the intention of folding, he checks behind.

River is a blank I check, he bets $200 into the $270 pot.

I can't see what I'm ahead of, but what is he checking back on that turn?