Maybe approach it as if you are meeting someone that you aren't related to. Of course it is awkward and I get being shy, cus I am too. but just make small talk, talk about things you like, how life's been what is going on with you, your betta passion, etc. I'd think any hard questions you guys might have might have should be asked at a later time when you know eachother better. I don't know your situation, but if you haven't seen him in 15 yrs, I am sure there are some difficult or emotional topics that might be best to be saved till a later time.
If you get to choose something, just choose to do something you normally enjoy, or maybe just go out to lunch or something :)
good luck Jooleeah! Let us know how things go! :)
1 Doberman Mix: Emmy, 2 DSH Cats: Rowan & Jacob, 1 Bearded Dragon: Rainier (aka Mista Lizad Man), 1 Eastern Box Turtle: Greta, Two 5g tanks: Alchemy (Halfmoon) and Custard (Yellow VT)