no sex : relationship advice plz

So I've been with this guy for almost 2 years now. lately for the past 5 months or so we have not been having as much sex as we used to. we used to do it a few times through the month and now I'm lucky if I get it once a month. I know it's not all about sex but I'm young (im 20 and he is 24) and I have needs!! if I could have it my way I would do it all the time with him! I'm scared there's something going on as to why he doesn't wanna do it much anymore. his excuse is he's always so tired. It's worrying me and I'm not sure what to do or how to bring it up without sounding desperate or something. any advice?