keeping it up?

My amazingly sweet boyfriend is 17 and I'm 19. I have been with a couple of other guys but I'm his first. He is the first boy that I have encountered this problem with and feel terrible not knowing how to help him or what to say. It doesn't happen every time but it has happened more than once. When he is able to finish the job the sex is great but sometimes we will be making out and go to have sex but at the last second it goes limp. Even though whenever it happens I assure him I love him for him he always gets all embarrassed and I just I could make it better. Help 😩