wont stay hard?

Anytime my bf (19) and I (18) have tried to have sex he can not stay hard once he puts on the condom and tries to put it in it will just go back to soft, i try to help but I don’t know what to do. I can tell it makes him feel bad but i try to reassure him that it’s okay and we can just try again some other time but i’m not sure why it happens! can anyone give me some tips or maybe know why? (also we do other stuff and he will get hard but then he doesn’t want to cum so he goes back to doing stuff to me and he goes back to soft)