How about I keep it to myself that I changed dinner plans and not tell my boyfriend?
Is there a reason that you don't want to tell him about the change of plans?
I don't want to have dinner with him.
It's rude to not tell your partner about a change of plans because they might be inconvenienced. If you don't tell him he might show up to the restaurant and be very confused.
I don't care if he's confused.
That would be a mean trick you will be playing on your boyfriend. It's wrong not to tell someone that you changed plans. You should want to respect your boyfriend if you care about him.