Mental stress can definitely affect hormone levels and desire. A tired man definitely loses interest in sex. I have a feeling it could be physical as well.
Let me share my experiences with you. I take plant fats - at least 1 tablespoon of olive oil at night and I always wake up with an erection. If I DON'T take the olive oil then I probably won't have an erection upon waking. Last night I took some maca powder and olive oil and this morning I had a nice erection that just would not go down. Maca is a great herb and is definitely increases sperm count and testosterone thereby increasing sex drive. Plant fats - olive oil, soybean oil,ect.- are essential and maca is optional but it certainly helps.
More importantly if he takes care of his body then he will have a good sex drive but if he abuses his body then he will be tired and have no interest in sex. That is certainly how I feel from my experience. A diminished sex drive is a sign first of all that something is wrong with your husbands body.