I want my bf to use his bday money to take us out to dinner
Does you boyfriend also want to take you guys out to dinner?
It is a bad idea to surprise him with something that involves him spending his own money.
You shouldn't expect your partner to use their birthday money on you. This may make them feel pressured into doing what you want. You should be considerate of what they want to spend their money on.
Ok.
If you want your partner to spend on you it would be better to ask him instead of surprising him. This is actually pretty rude and may send the wrong message. You are better off just being honest about your intentions.