Probably his father obtained Portuguese citizenship through naturalisation etc, so he isn't eligible for one. If that's the case, he can get EEA family permit as family member (direct descending line) of an EEA citizen. He needs to be under 21, but otherwise there are few conditions.
As I said, apply for EEA family permit for UK. To apply, your father needs to be moving to UK with you, and attaches a letter stating he is going to UK and requests EEA family permit for you so that you can accompany. You need your passport, certified copy of your father's Portuguese passport or his original national ID card and your birth certificate showing your parentage (professionally translated into English). There is no fee.