I don't think you need to worry about him being interested in someone else. It sounds like you're just going through a phase. Here in a few years, you are going to hit your sexual peak.

You know, my husband and I have been on and off for the last 11 years but going strong on for the last 4 years and since we got together 11 years ago we have gone through our phases of sex twice a day every day in the beginning to once a week to once a month back to 3 times a week (it's never been back to that twice a day though lol) We even went through a time where we went without for 4 months. now it's about where yours is, once a month or once ever 6 weeks or so...I personally hate it but that's just me and he's ok he doesn't mind it. I went through what you are going through thinking that I was less desirable or that maybe there was someone else, or whatever...but in the end it was just that life happened to us. We have a different life than we did 11 years ago. We are hard working with a child and are more tired and don't have as much free time or there is just always something going on. I have my moments where I don't care it doesn't bother me and then I have my moments where I would like to do it more but when I sit and really think about it...we just don't have the time or energy for it like we used it. It just sounds like life has happened to you guys and there are other things going on. At the same time I think everyone goes through a point in their relationship where it just isn't as important to keep you together as it used to be because your relationship is based on something so much bigger than sex and that is just the plus to it. I am sure it will all work out for you.