I would not be able to tolerate that choice. However, you already know that he does not tip, so I am not sure why you thought it made sense to agree to go to a restaurant. Mentioning the issue at the table as the check is being paid is very confrontational and did not result with the server being properly compensated.

Your boyfriend's thought process is illogical. His decision to purchase birthday decorations for a twenty year old woman has nothing to do with the basic social contract of tipping 20% for adequate service. That is how restaurants work. His assessment of what the server had to do has zero impact on how tipping works. He is wrong and his error reveals that he is childish, self-involved, and not able to think clearly.

But he is your boyfriend and you decided to go to dinner with him.

If you continue dine out with him, you need to surreptitiously leave a full 20% tip so that the server always gets at least that. Bring cash and leave it on the table just as you are leaving or hand it to your server privately while you pretend to use the ladies' room.