spice up your sex life, or maybe he's cheating(hope that is not it)
ask him about it. talk to him about new things you would like to try. try different toys, different lubes the his and hers one is rly good :) also try some role playing, or dress up, costumes can be fun
I'm not a guy, but maybe you should step up and be a little more mature and ask your dad about it... if your that concerned you can talk to him and have him make you a doctor appointment... and you can tell him not to tell your mom, cause that could be a little embarrassing. but your dad should...