Dear Therese, 

You are ending highschool and intend giving a year. You like this boy, but as regards the faith he is not at all on the same wavelength as you because he has had no one to show him. 

Whether you will get anywhere or not by pushing him to go to Mass depends on a number of factors, among them how you do it and his temperament. He seems to be easygoing enough so he may not mind you reminding him, but that doesn't mean he is going to budge. 

I think you have an opportunity now that you are going to give a year to set some things straight with him. Tell him you don't want to be a nag but at the same time you see he is missing something in a very important area in his life. Tell him you would really like him to look into it but you don't want to tell him how, or do his thinking or praying for him. Nevertheless you think that he will never really understand you or the choices you will make in the future if he doesn't have some idea about the faith. 

So really you are trying to do several things simultaneously: you want to respect him and his freedom, but you also don't want to be silent if God wants to speak to him through you; you want to take him into account, but you also want to make sure you keep your priorities, and you also want him to know what is truly important for you.  

God bless.