You're not going to want to hear this but I don't think you can without him resenting you. I also agree its not fair to you. He's having an earlyish MidLife realization that he's not getting any younger, probably prompted by his dad dying. You might be able to ask for some limits to the surfing time, to balance it out. Say that to him, but not a full ultimatum. I get that you lose attraction for a man that is doing something childish in your eyes, so add that to the conversation. You may have to give in a little to not have him rebel like a teenager and double down on this activity.
Also its within your right to say this is a dealbreaker for you, too. You didn't marry full on beach man galavanting amongst beach babes, after all. I'm sure the thought of those girls aren't helping your image of him, either, be careful that you aren't using the stopping surfing thing as a redirect of any insecurities you might have with your self image. That might be or might not be, I mean I'm just a 41 dude on the Internet dispensing advice to people generally younger than me, the same people I always feel give the information in the posts that actually written different ways could be their own advice. What could I know? ;) Take care of you, too.