If you love him and want a future with him then maybe sitting down to have a conversation with him would be best. I'd say to him, that you know the two of you agreed to be "friends" for now but with him professing his love and want to marry you, is there an idea of how long you will continue to be "just friends." You can be honest with him and tell him that you do love him a great deal and want a future with him but you're also not going to wait around forever on the slight chance that things don't work out as the two of you hope. You can share that you'd like to see things progressing to the next level by (and give him whatever time frame you feel comfortable with; 6 months...) but until the two of you clearly state that you're no longer "just friends" if opportunities present themselves, you will be dating other people.
You can reiterate to him that you're about to turn 38 and having children become increasingly harder as women get older so the window of opportunity is narrowing and then leave the ball in his court.