Knowing you have feelings for him and knowing he is involved with someone...really doesn't lend itself to seeing this person this weekend. Although I'm guessing you probably will see him in some capacity...it really comes down to setting your boundaries...and letting him know what they are...

Let's say his behavior is crossing the friend line...you tell him that you don't believe that its appropriate considering he is in a relationship. What's crossing the line? Well think if you were his girlfriend, what things would you not want him to be doing with another girl?

I'm guessing he will pretty quickly get it...if he doesn't then I trust you will realize he doesn't respect your boundaries...which means he is not respecting you...