It sounds like he may be letting stress and issues in his environment to effect his desire to have sex. When sex gets very infrequent this can be indicative of emotional issues in one partner, health problems, or just stress related emotions that interfere with the desire to enjoy sex. It can be the relationship but most of the time it is not. He knows why he is disinterested in sex and that is the problem to target. If he is consumed with problems overall in his life then work on this as a couple. The issues have to expressed and resolved though. You can use this as a way to strengthen your bond as a couple. He has to communicate his thoughts and feelings. You can also work on sex by working on intimacy. Schedule date nights that encourage time together without talking about problems that you both are having. Quality time spent on romance will increase time spent on sex. Make sure you have this quality time regardless of what else is happening. Also read my favorite on this subject Mars and Venus in the Bedroom.
You can communicate your needs but first just have fun together. Sometimes if you focus on the lack of sex it puts pressure on the partner. Don't allow this issue to govern your relationship. Learn how to communicate about sex without making this the sole focus.
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