Is your boyfriend my husband? LOL! My husband does the same exact thing. It doesn't have to be the same restaurant- it is any restaurant, various foods, drinks. Again, he doesn't sneeze after eating at home, nor at the restaurant..........just in the car after leaving a restaurant. The only time he sneezes in the car- after leaving a restaurant.
I'd be kind of willing to experiment with this. Have him go to the restaurant with you and not eat one single bite or don't drink anything. At least you're narrowing it down - if he gets home without sneezing, it's something in the food or drinks that does this.
Obvious thought is an allergy to something. If you always go to the same place for dinner, then that may be the trigger. Alternatively, are you sure the sneezing is just related to the dinner, or when he is in the car? There may be an allergen in the car. Another more obscure idea is that somehow this has become some sort of conditioned response from him. He expects it to happen, so it does. I remeber once I got nauseus from a horrible lunch before inside a library. For years after I couldn't enter any public library without feeling sick. I'm fine now. If it isn't an allergy but a learnt response, maybe just being aware of this might change things. But I am not an expert, just thought your problem was interesting!