Many men watch pornography since they are visual creatures and like to get turned on by what they see. In addition, it sounds like your boyfriend has a high sex drive and so the times you are not having sex, he is getting turned on "pleasuring " himself as a release. It sounds ,though, like he may be dealing with an addictive element to the porn since even after having sex he is still aroused and wanting more.
What you may want to consider is whether he is still excited and wanting to get off again when you have finished making love and if that's what's driving him or if, in fact, he's just still into sex even if it's more passively as a viewer of it. Rather than tell him to stop it or that he can't do it, which has lead to his lying about it, can you try to talk openly with him as to the nature of his habit? How long has it been going on, can he get off without some degree of porn? This way, you can gain a better understanding of what his needs are and then both of you can attempt to find some balance and compromise as to the degree he watches it.
Also, if you can accept it as a sexual need of his and not something that takes away from what he shares with you, it will make room for you to even share in the porn watching with him and could actually rev things up for both of you leading to something different and exciting in your sex life. When it comes to sexual preferences, it's hard not to react to them but it's really important to try to make room for them so your partner doesn't wind up feeling judged and not accepted. This is when they tend to lie and or it will create distance since they will feel they can't really be open and honest.
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