Maybe he can't remember? I ask my kids all the time what they had for lunch or how school was and they say I don't know or I don't remember! Makes me wonder what they do all day. Also, maybe flat out confronting him, and saying I know you didn't have that because I have a menu of what is served. May just be the age too. Easier to lie then to have to think! I wouldn't be to worried about it unless he starts lying in every aspect.