Boyfriend 'Jokingly' proposed to me?

Okay so this happened last month. We've been together about six months now. We were discussing marriage at a cafe we were at (he is a bit anti-estqablishment in his views btw) and then out of nowhere with a grin on his face said "Hayley, will you marry me?" I knew he was saying it jokingly but I almost choked on my drink and couldn't form a response I was so shocked. So he asked me a second time "Hayley, will you marry me?" and I almost choked a second time. Then I said "ask me again another time and there's the matter of the ring too" lol and he responded with something like "whoops I got myself into this one" (I can't remember the exact words but it was something of that meaning). And then we just laughed it off.

My question is, do you think even if it was meant to be a 'joke' that it was serious on some level? I told my mum and she said it does sound serious and that I should've said yes lol. I mean would he say that if he hadn't have thought about it even seriously just once? Or am I looking too much into it? We make a lot of marriage jokes between us too and my mum thinks it means he does one day one to marry me (and I definitely want to marry him too).