I'm not sure if there are any State laws which may prohibit the lending of firearms, but if he meets all of the federal requirements, I say it's good to go.
I assume he has no Wants or Warrents, no history of battery or assault, has not pleaded temporary insanity, nor has he plea bargained out of any prohibitive charges since he was awarded a CCW by the state. It sounds like you have your legal ducks in a row regarding the letter of permission. You've got your name, his name, date, serial, your signature and his. I think you are both legally covered.
That is more than enough for such a situation down here in AZ, but maybe somebody more familiar with Mass. can chime in with their experiences.
After thought, one thing I might add to the letter of permission is maybe a statement regarding WHY he wants to borrow your firearm. "I am involved in a legal action regarding a large sum of money which is owed to me, I fear retaliation from the defendant for........" Something like that may cover any forgotten I's or T's.
"This country, with its institutions, belongs to the people who inhabit it. Whenever they shall grow weary of the existing government, they can exercise their constitutional right of amending it or their revolutionary right to dismember it or overthrow it." --Abraham Lincoln