You’re not alone in the confusion about the booster recommendations for people with compromised immune systems. I think there’s always going to be a mystery for any of us who have an immune system that isn’t as robust as our ‘factor installed’ version. We’re at a distinct disadvantage for not knowing how our bodies will react to something even as minor as a common cold. So then toss in this unpredictable and constantly mutating Covid virus and no wonder we’re confused. We all want protection and have no idea how many shots it will take or even if they will work for us on some level.
There’s a lot of unknowns yet about the effectiveness of vaccines on immunocompromised patients and it’s an area that requires more research and remains an evolving field.
In my opinion, since your husband has already had 4 doses of the Moderna vaccine and the Evusheld, I’d really talk to his oncologist before proceeding with any more. Since he had Evusheld it won’t pay to have a titer test as he’d likely show he has some antibodies. The problem is that there’s more to the immune systerm’s reaction than a number count on the titer. There is also the action of the T and B cells which might have some level of underlying activity. So I guess until we have better answers it’s still up to us to avoid infections by masking up, hand sanitizing, avoiding crowds, etc.. It’s a frustration, for sure! I think we’re all tired of the lack of normalcy.

Have you talked to your or your husband physician about another booster?

@jam5 My oncologist told me just last week we will "play it by ear". I received Pfizer vaccines 1/23/21 and 2/14/21, boosted with full dose Pfizer 8/19/21, and Evusheld on 2/17/22. He feels we are doing all we can. He also encouraged me to consider my health when moving about this crazy world, and decide what will be best for me. I am actively on chemo for my mm, plus additional health issues.

Masking up, hand washing, avoiding large crowds, keeping stress levels low are key on my list. While the FDA will provide guidelines, I agree with @loribmt that your medical team, who knows you best, will be the best source to help you make a decision.
Ginger

I am immunocompromised. I had my fourth full shot last month and have been told to get a full fifth this week.