And it's especially unusual for a man to lose interest - usually it's the other way around. So a couple of things here. Get him to talk to you to get them off his chest. Also take time for sex - get him to relax with massage, nice smelling oils, maybe a bath together first etc. If none of this work, you have to have an intimate talk with him about your sex life because if it's already fading, this relationship won't last.
Sex is the glue that holds couples together.
I don't think this is normal. When my Boyfriend goes down, it doesn't do anything for me. A lot of women don't have orgasms when they have sex. It can do with a lot of reasons. Can be the lack of experience and skill of your partner who isn't sure how to give you one.