need help/opinion

So here it goes... My partner and I have been in a relationship for about 5 years now. I'm totally in love with him in every way. The only thing that we really fight about is sex though. He works really hard compared to me, so I understand not wanting to have sexual relations that often. Over the past years it has dropped to once a week for the most part. I've tried new things (had him tie me up), really haven't tried anything else yet. I want to get him more involved but he seems to just shut down every time I bring it up. We fight kind of constantly about it and he even mention how a previously relationship did that too him too, so he knows what I'm feeling.We decided to do an open relationship cause of that, so I'm still getting use to that. I feel like I would be cheating on him though. I know he jerks off almost everyday on his phone and he looks elsewhere, not sure if he ever connected with anyone yet, but he is looking. I just wish there was a fast fix to this or if it is even worth trying to fix. Just hate not being able to talk to him about how I feel.Thanks for letting me rant and get it off my chest... Any thoughts are welcome.