Boyfriends Engagement Ring

My boyfriend and I have been talking a lot about getting married and engaged. Actually we've been talking about it since day one. We know that when we decide to take it to the next step, it won't be a big elaborate surprise or anything and I recently found out he had one custom made for me. He also recently told me that he wanted one as well, if I wear one then he wants to as well. I'm okay with the idea but I'm not sure what his size it. He doesn't wear any rings now so there's no way to size him except dragging him in which I don't want to do. I don't expect to give it to him until he proposes so it can be a surprise for him. Help!