Erection loss and inability to come

Hi, I've been married almost a year and my husband and I have always had a great sex life, we're very in love and happily married. We've always been able to have sex for an hour at least which has been great, but lately he has been losing his erections during sex, and has not been able to come in me anymore.... He's 28 and I'm 23. I'm horny all the time and really want sex but then we just feel exhausted from doing everything to keep him hard and make him come. He gets rock hard, but after a while it just drops off. We really want to start trying for a baby, but it's so annoying now because he can't come and has trouble keeping erections. I've tried everything I can to turn him on, but now am wondering does anyone know of supplements or a tablet that he can take to give him good erections that will last and make him come?