How do I get my dad to stop wearing speedos?

I am 18 and my dad is 52. We live in Sydney’s northern beaches and very often go to the beach. I think that he shouldn’t wear speedos because a) they’re very revealing and b) he’s old. I have asked him several times whether he could please start to wear shorts like me but he just flatly refuses and won’t have a bar of it. He says that he has worn speedos since he was young and never once wore anything else. He owns four pairs of swimwear and they’re all the speedos. How can I get him to stop? Also I should add - he isn’t out of shape, he is very much in shape, but I think that he is too old.