I echo all those who say be upfront and honest about where you’re at. And that it doesn’t have to impact whether or not you remain in a committed relationship. If the marriage thing is a dealbreaker, let him be the one to decide. You’re doing nothing wrong by being in a relationship with him, whether or not you wish to marry.
I wonder if culture in the US places more importance on marriage than British culture? Here it’s far less of a deal for two people to live together and remain unmarried. In fact, I know many people who only decided to get married after decades together, presumably partly because legally it’s a good idea for life partners to have that piece of paper. But also because people are cautious over marriage, rightfully so.