fold the flop.
also, dont confuse laggy play with stupid play. lags like me in cash games will call and raise a lot preflop (I'm about 33-36%), and make maybe one continuation bet, but other than that a good laggy player isn't bluffing turns or rivers unless he has you on a draw (i.e. you simply call his bets, and he has a read on your prior play). Many good laggs only go all in with top quality hands (sets, flushes, straights). Think of them as players who like to see flops to hit big hands and bust others, and afford all the expensive calls from all the wussy folds from other players (at weak-tight tables, I can make money with a2c and cbets). Re-raise their flop and pre-flop bets to put a cooler on their style, but if they bet turns and rivers its time to shut down (although on this hand, hard not to after turn).