[Larude]

Hmmm as a microstakesplayer I would let ATs even on the button if I am not the aggressor (or if the pot isn't a limped one). After your call on the flop you have decided I hope that he hasn't got 22, TT or AA so you have to call the river. Villains line is really weird although on this ultradry board he might take this line with 22, AA or TT; he checks the turn in the hope you have AT, 22 or maybe TT and fire again (if he has AA or TT himself), so he can call or check/raise. On the other hand this also might be a villain who thinks you are messing around after so much 'weakness' on the flop and he decides his AK or AQ can go to a cheap showdown (through confusing you and minraising you). 51 could really be a blockbet, it wouldn't surprise me if he had AK or AQ. As said after your call on the flop a call here is mandatory, maybe even for information in later hands.