I am really sorry to hear that your sex life with your husband has suffered as a result of his stress at work. As you have suspected already, stress can definitely have a serious impact on sex drive. In addition to the stress, fatigue may also be a factor.
Unfortunately there is no easy solution to this problem. One thing that can work for many couples is to start trying to find ways to make scheduled quality time together. Often one of the first things to go when things get busy and stressful at work is scheduled time with your partner. Having sufficient time together is the first step towards reestablishing a pattern of intimacy more like you used to have.
Additionally, other stress relieving tactics may help - such as regular exercise and healthy meals. You should work with your husband to make sure your sleeping habits are as regular as possible. If possible, your husband should avoid alcohol and caffeine in the evenings, as these can impair both sex drive and also disturb sleep, which could further contribute to fatigue.
If these approaches do not work, your husband should talk with his primary care doctor
as a first step toward figuring out what to do next.