I don't know why he's on the list for vaccination already, anyway, but if he would get AZ there's indeed a high chance for a young person to have side effects like flu-like symptoms for some days - nothing serious / harmful, but enough to take him out of training for a few days again, and I can understand that that is not what you would want right now. (I've heard of quite many people with side-effects after the first shot.)
And I think if he says I'm not taking it now, but I will after the Giro, that doesn't do harm to the vaxx-image.
He's on the list because Belgium recently decided they'll prioritize vaccinating the Olympic Athletes.