18 person $20 dollar SNG

15 players left w/ 8 at my table.

I'm on the button w/ 1 limper and the blinds are 25/50 I think.. I have ... I raise it 5x and both blinds fold. Limper waits a few seconds and calls.

Flop:

limper goes all in w/ a huge number of chips...Cant remember how much he had now... He was second in chips at the time w/ some loose play... I'm like in 5th or 6th place w/ a tight image.. Only shown big hands and preflop uncontested raises w/ AK.....

What do you do???