You are 26 and your son is 17? Something there does not add up.
One thing is rules, another thing is relationships.
Your son, living in your house, needs to follow your rules. Rules would normally address things like drug/alcohol abuse and church attendance. I assume he is breaking no rules in this relationship. If he is in violation, those things need to be first addressed.
The most important result is the relationship you maintain with your son. Do you spend time with your son? Can you spend time with his "girl friend"? Though there are no formulas to live by, the purpose of the entire "girlfriend/boyfriend" dynamic escapes me. Choosing a mate, in my culture, is a family affair, not an individual one. When teenagers, or even adults, are having a serious relationship with someone of the opposite sex that does not contemplate marriage as the end, I wonder what the purpose of the relationship might be. Do you share this point of view? If not, can you articulate your point of view?
Then, can you articulate your point of view to your son? (This would be relational sharing, not setting new rules). Then, can you articulate your point of view to your son's girlfriend?