Crystal,
I agree with Mike and Batch, it's always best to have it checked out by your husband's GP.
Personally, I have experienced that dizziness and lightheadedness while in a CH.  I'm also taking the vitamin D3 regimen but, what I believe is happening to me is not related to the D3 regimen.

I'm a massage therapist and through the years whenever I get into cycle I notice I also get very, very tight muscles on my left neck and shoulders (same side as my headache pain). For some reason CH does this to me. My Doc joked with me and said that the ultra tight muscles were "along for the ride."

Very tight neck and shoulder muscles, especially ones that attach to the skull, will pull on those cranial bones ever so slightly disturbing the organs of balance inside the Temporal Bone and that (at least in theory) is what causes my dizziness. It's a rare occurrence, but it does happen, even to people who don't get cluster headaches.
 
When I get a deep tissue massage on those specific muscles that effect diminishes or even goes away, although it's always temporary. Another CH comes along the next day and those muscles get tight again and, well, there you go....

If your husband's GP gives him a clean bill of health, see if you can get him to give deep tissue massage a try. Be sure to find a therapist who has a number of years experience in deep tissue massage and/or neuromuscular therapy.
It won't do any harm and might even help (though temporarily as I said).

Steer clear of relaxation massage or spa style treatments, those won't help at all and might even aggravate the situation (I know it does for me).

Massage will NOT stop a cluster headache once it's started, it will not stop a cluster cycle, it will not alleviate the pain from a cluster headache and it will not prevent cluster headaches. But it may give some much needed, though temporary, relief from the muscular tightness and pain that can accompany CH.

I will say this: This is the easiest and mildest cycle I've ever had and I have been diligently taking the D3 regimen since coming into cycle (with some modifications and extra coaching from Batch). The headaches I have had during this cycle have been much less painful, shorter in duration and more manageable than compared to previous cycles. I believe I owe that to the D3.

I hope this helps.
Gary

A respected neurologist from Ireland recently told me that more than half the patients referred to him with suspected Hemicrania Continua had, actually, their problem originating in the neck, a Cervicogenic or a Dysfunctional  or a Hypo-Mobile Cervical Spine.

A common playmate of this condition is Cervicogenic Dizziness. A simple X ray (not MRI) should indicate whether that diagnosis is worth further investigation (but unfortunately not rule it out)

If this is / might be the underlying problem then the person performing the deep massage should be aware of the possible connection as further harm could be done.