How to tell........,??????? ?

Ok so I've been going out with my boyfriend for 8 months now and my dad doesn't know and I really feel like lettin him know because I feel like he would let me go out with him more but at the same time he would be really mad and disappointed. I'm 15 and to him a good age to start dating is 18 and all this time it's been mymoms and I secret and I tell her I want to tellmy father but she says it's not time yet. But when will it be?? And how can I tell him?? Help :(