My best guy friend and I have had a "thing" for about 7 months... We can't date but want to stay friends.?

He is Mormon and I'm Jewish. His parents hate me for ruining their sons life (they're extremely strict). We've been trying to keep our relationship strictly platonic but it's been with little success. Not only has our relationship grown physically, he's also my best guy friend. His parents have banned me from being at his house because I'm not an LDS member. We want to stay friends (partially because we're in the same cles and clubs). He'll be leaving for his mission after graduation... But how do we stay strictly good friends until then?