Thank you so much for your excellent advice.
He said it in the beginning of my visit after I picked him up from Kindergarden.
I told him there are many names for God and that I just call him dear Papa in heaven but that I don't believe in Jehovah (it is confusing, but I'd like him to know that I don't believe in Jehovah).
I told him that people believe in many different things and gods or that also many don't believe in god at all and that he is free to believe what he likes.
After that for the next 2 days he didn't say anything about Jehovah but enjoyed our time having fun, playing, reading stories etc.
And he didn't attend the convention because he was with me ;-)