Don’t worry about a pushback or stepping on the toes of his current doctor. You have to be your own, in this case your husband’s, advocate for healthcare. If you’re not feeling all warm and fuzzy about his current doctor, it’s within your right to be that squeaky wheel and seek an appointment elsewhere.
Mass General would be an excellent option. Larger hospitals usually have teams that work collaboratively to provide the best care for their patients.

I found their Request an Appointment page: https://www.massgeneral.org/appointments-and-referrals#:~:text=Call%20our%20patient%20care%20team%20at%20617-726-2000%20or,care%20team%20and%20manage%20your%20appointments.%20Referring%20Providers

Having a second opinion will give you peace of mind. Will you give it a try?