Hypochondriac or a mood disorder? Mood disorders(depression, general anxiety disorder, bipolar) could manifest themselves as physical issues such as low energy, decrease or loss of libido, a desire to be alone, among other somatic expressions. I speak from experience, having bipolar illness. As my illness has worsened over the years, I've become more withdrawn, moodier, more anxious, along with more physical issues. My wife also has mood problems, though she's been self-medicating for decades with alcohol.(Addiction and mood disorders run in her family.) And, like your husband, I'll suddenly have an upward mood shift, which ramps up my libido, then I'm back to my baseline of moody and sad.
You can't make your husband go to a psychiatrist or have him discuss it with his primary, but perhaps you can have a heartfelt discussion with him. Being gentle is always the key when you have a heart to heart with your spouse or s/o.