Should I leave him, even if I don't know the truth?

My boyfriend of 7 years just confessed to me that he is "on occasion" snorting percocet. His reason for confessing? Because i found a bic pen (ink removed and cut in half) and a a small grinder (for apparently crushing pills) in his coat pocket. I really feel like he is not telling me the entire truth. I had suspicions that he was using drugs but of course he denied it and I could never prove it. He knows that I have no tolerance for people who use drugs (except for pot), which is his reason for not telling me anything. He reminds me that when we first started dating that we both (socially) did drugs. But we were teenagers and it was the culture of our high school, but were adults now and that SHOULD HAVE been a faze! I don't' know what to do at this point, my gut tells me that its more serious then just percocet, but again I can't prove it. What should I do?