Hi Jam,
Like your MD in NJ, Mayo Clinic is not recommending booster shots until they are approved by the FDA. Here is the official statement to our patients
"Immunization boosters have been given in many situations and have generally been considered safe. However, COVID-19 vaccine boosters have not been thoroughly studied. We recognize the support for COVID-19 immunization boosters by some experts in the field, but an official recommendation from the FDA regarding COVID-19 boosters in immunocompromised patients has not been released."

I know you want him to be protected from COVID. Along with being fully vaccinated (2 shots), your best protection includes wearing a mask, distancing and washing your hands.

@loribmt may have more to add about immunity levels post vaccine and living with a compromised immune system.

It sounds like you are doing the right thing by seeking a third opinion about treatment options to be sure to know all the possible options. Have you considered asking about clinical trials that may be available for your husband's condition?