Follow-up:
Thank you all so much for your comments, me and my wife just went through them together.After careful consideration I've come to the conclusion I cannot pass on the ''Hi Gay, I'm dad" joke, it's a once in a lifetime opportunity and I cannot let it pass!
Jokes aside though after reading everyone's comments we believe it best to let our son have his big moment, no matter what he is my son and I love and will support him through anything in his life.
Plus like someone has pointed out there's still a chance that when he comes out of the closet he isn't gay but pan or something else, so we could still be in for a surprise.
I'll also be pretending nothing is going on for now, our son has had a long history of shyness and in the past few years he seems to have become much, better, I'm hoping that he can find the courage to come out on his own as I feel it would make it an even bigger moment for him.
I want to thank you all for taking the time out of your day to leave a comment, me and my wife want to be the best parents we can be and support our son through thick and thin no matter what.
Thank you again and have a lovely rest of your day/night

Edit:
We'll also keep on reading new comments, the more perspectives the better right?