if he is too ill for commercial air travel, and Mass General is your only acceptable choice, there are air ambulances you can charter. I don't know the cost but it will be in the tens of thousands of dollars.
Google private medical jet for options.
Another option [depending on how sick he is] is to hire a nurse to travel commercial with your husband.
HERE is a company that provides that service
Lastly, be sure to add into your deliberation the additional stress on his body of this long journey. Are you certain the additional advantage of MGH outweighs the disadvantage of the long trip. Only you can make that judgement.
Best wishes