What do you think of me not going to tip my SO when he serves me at his restaurant?
But that's mandatory. Is there any reason not to?
I don't think I need to because we're in a relationship.
Regardless of your relationship with him he is still serving you food at a restaurant. You should tip him like you would any other waiter.
If he was my friend I would tip him. But he isn't a friend, he's a SO.
Even if you are in a great relationship to do such things, he is still in a formal job setting and you have to be treated as any other customer. How are the other servers going to feel if you don't do your part in tipping? How about you just tip, and he can use that money to buy you a surprise?