CHL - that is all you can do. And I give you tons of credit to be able to step outside yourself, hold your tongue and just be there for him.
I have a nephew that is bpt'd and he is currently pioneering. He has an LD (dyslexic) and has struggled his entire life. JWs give him a community and something he can feel "smart" about because he does not feel that way about much in life. So I can understand his need to fit in. He visits me and we rarely talk about his religion. The only thing I do is tell him I love him unconditionally and will always be here for him where ever he is or is not in his life.
I can tell you that it was the hardest thing in my life to hold my tongue when my nephew told me he was getting bpt'd and when he tells me now of his plans for his JW future. But I had to learn the road I wanted to take and it would not have stopped me if a non jw family member tried to stop me. So the only thing I can do is hang back and just be there with all the love and support I can give. Til the day he realizes this is not for him and then I will be there to help him deal with that.
Unconditional love is an amazing thing and you are a great uncle to be able to give it out.
Peace - L_G