Can I tell my PhD supervisor honestly why I want to leave him
My suggestion is that you talk about all of the reasons you are moving to the other university, as opposed to leaving the current one. These reasons could be to do with relationships (e.g., family) or future opportunities (e.g., a postdoc) or being paid whilst you are currently unpaid. If you have to give a reason why you are leaving your current position, the least-harmful choice seems to be to mention that his lab will close and that you chose to study with him in the hope of staying within his wonderful group.

The choice comes down to the problems of being honest (which does not imply you should be dishonest). If you are honest, then because he will retire even if you magically changed his world-view, then the impact would still be negligible. However, you may burn bridges and although a small probability, the professor may take retributive action.