I definitely understand where you're coming from since my gut reaction would be to do the same to diffuse the air and try to make a joke and I also would've appreciated that kind of reaction when I came out, since i don't care for sentimentality (i'm bi, btw)

But coming out can be big deal for a lot of people and combined with the fact that you seem to be aware that they are more sentimental, you should have gauged the air i the group chat accordingly. But don't beat yourself up over it. You're still in highschool right now, plenty of time to grow. Not everyone reacts to emotional stuff the same and that is something we have to learn to fit in with other people better. Right now just make sure that you make up with your friend . Tell him that you do care for him and this was just your way of trying to diffuse the situation and that you were sorry that it came out at the wrong time in the wrong way.

Acknowledge his sexuality and then that's it. Also, make sure he knows that you do think he's pan and not gay.(i get that it's a bet) People tend to assume sexuality of pan guys as gay and pan girls are straight ( you've no idea how many comments i've had to hear ) . so just make sure he knows, if you don't want to strain your friendship