I'm afraid my advice is to say nothing and just go ahead without him. My daughter met a guy online when she was 16 and it was a waste of four years of her life, they met up twice in that time and her life went down the toilet and never recovered even though she is now 28. She blames me for everything because I questioned what she was doing during that time when her grades went down, she refused to go on a once in a lifetime school trip to Japan because she wouldn't be able to talk to him, not to mention the Disney trips because I refused to let a virtual stranger join us, at my expense, no doubt.
Time does come when family traditions like vacations to Disney do start coming to an end, that is a fact of life. I remember the first time I went to Disney on my own, 10 years ago, because she didn't want to come, sorry, but, best trip ever! First time it was about me, something that never happened before.