2012 f150 tire max tire size on factory wheels?

[GT500662]

My Son just bought a low mile 12 f150 XLT super crew with the factory 18 inch chrome wheels. He is wanting to change out the stock tires for some MT ATZ like I'm running on my 12 F250. But is not wanting to lift the truck or modify it in any other way. Just wondering what size he can get away with running without looking silly. Thanks in advance for any information given. Jon