We did this on our 2003 trip for 2 days a friend joined us which wasn't expected. When we checked in at the podium asked if we could add 1 more and there wasn't any problem.
We are a family of 6 though and just squeezed in and added an extra chair at Cindy,s for lunch. Same at Rainforest AK.
Also was prepared to go somewhere else if they couldn't.
I would call Dinning to add him if it is a must do meal or if he is going to be there for your whole trip and eating with you every meal.