Mazel tov on your simcha! May it be one of many many more!
The Rav’s level of derech eretz is not anyone’s business, that fact that you extended an invitation is a nice gesture but that is where your part ends. If the Rav would have come and only stayed for 5 minutes well some would say that was rude. So The Rav is in a no win situation. And if you already know that this is his “minhag” why is this an issue?