If you are not getting there, speak up. The earlier you let him know, the better for your sex life. Faking it helps no one. If he knows you are not cumming, you both can figure out new and fun ways to make this happen. You shouldn’t have to pretend the rest of your life.
There are some women out there who will rather receive than give. They get a selfless lover and they have a problem giving back. This is unfair. If he is giving you all that good feelings, be sure to return the favour.