Husband has low sex drive at 24...any advice?

So my husband and I have been married for over 4 years now. He’s 24 and works for our county. I’m a SAHM to a son and TTC #2. He works long hours and A LOT. He picks up a lot of overtime and also has a side job. He doesn’t have a lot of days off at all. He always tells me he is just too tired and I respect it because he is understandably tired and if I were him and was too tired for sex, I would want him to respect that as well. That being said, my sex drive is through the roof! I got off birth control almost 18 months ago, and ever since then I feel like a new person. I always thought I had a low sex drive, boy was I wrong. About the time I got off BC, my husband was getting out of the army and trying to get his now current job so he was stressed and tired. We are having sex probably 3-4 times a month and I’m starting to feel resentful and unwanted because I’m being turned down so much...I do take care of myself and am in good shape. I’m not sure what to do but this work load will not be lightening up and I’m desperate. Does anyone have advice?