Player tells me to check it down, I am confused....

[mgsimpleton]

given that he had an overpair and played it so passively, which is what you can assume (he really was scared and didn't want you to bet), the turn bet is actually profitable.

1/2 the time you lose 225, but when you bet 225 and the pot is now 750, if your river bet of say 500 gets paid off every time, it's profitable because had you not bet the 225, the pot would only be 300 then let's say villain would only pay you 200, 2/3 of that pot. so you've increased your profit by 300 the times you hit and lose 225 when you miss, and you hit roughly half the time.

obviously this is over simplified since he might not call you when you hit the T or the 7 or you might bet less, but you get the point... you can play with the turn bet amount to see what the most optimal is given he check/calls AA. (in a way, the idea that he check/calls with A9 on the turn and checks the river and you check behind unimproved and the turn bet is purely profitable "offsets" when you hit a scary out and can't get paid off as much)

[Ass Master]

My guess is that the villain is one of many online NL players who are unaware that there are differences between tournament and cash strategy. I think he wants to check it down so as to increase the probability of "eliminating" the all-in player.