Okay you and him are not "officially dating?".. So there is no need for you to explain to him why in detail you don't drink.. I also wouldn't advise you ordering something just to fit in or appease him.

I would caution you to not feel the need to explain your bipolar to him until you can see a future with him.

He should like you for YOU .. You shouldn't be worried about making him like you.

Stick to your decisions about how you want to live you life.