First of all huge hugs.
My feelings....if it was me....I would call him.
I would let him know how glad you are that they want you with them, but you don't drink anymore and aren't comfortable being around alcohol.....I would imagine he will completely understand.....I would suggest an alternative for the four of you on another day to celebrate your friend's 40th.....perhaps you could offer to take them out for a lovely lunch or buy a ballooning trip for the four of you.....that would be awesome.
But that's me......I am comfortable telling people I don't drink and do not like to be around it....especially in a situation like a boat where I am trapped. I don't tell them that part.
♥