QUESTION REGARDING FILLING FOR TAXES.?

No, he will not get introuble. Yes, you can file jointly. The point of a W-4 is to calculate how much taxes comes out of a paycheck. Claiming single will just take more takes out that needed because single people pay more taxes. You will get that money back when you file your tax return jointly. His ex would never know what he's claiming at work unless she sees his check stub.