I caught my partner watching porn early in the morning and when he went to bed.
How do you know he was watching porn?
I could see him watching porn on his phone.
While he shouldn't hide things from you, you shouldn't invade his privacy by going through his phone. It will make things worse and lead to a fight.
He lied to me and he will feel bad. He should learn to be open with his partner.
Perhaps he feels he can't talk to you about his enjoyment of porn. Maybe you should open a line of communication about the topic.