I am trying to understand this correctly but I am quite puzzled @EE1980 I'm sorry if I am just being thick.
You were just asked
"to ensure will reflected his intentions"
So did you just tell your FIL
"Yes, it reflects your intentions"
without asking your FIL any questions to make sure that it does indeed reflect his intentions?* *
Without delving further into your FIL's thought processes when he wrote the Will, how can you possibly know if the Will reflects his intentions correctly?
In your position I would consider myself to be not doing as my FIL requested if I did not say to him something like this after reading the Will:
"So your intention - insert name/FIL/Dad - is to leave everything to - insert BIL's name - ? If you can explain to me why you do not want to leave anything, even something of purely sentimental value, to - insert DH's name - then I will know whether the Will reflects your true intentions or not."
You can say that to him politely, and without it sounding controversial, as you are doing exactly what he has asked you to do. I don't see how your DH could be upset by you fulfilling your FIL's wishes. You do not need to say that your DH is hurt by his dad's Will, just stick to the requested input.
Assuming that your FIL does explain his thinking - including the fact that he is not even going to leave his second son his much loved watch or such like - you and your husband can then discuss whether your DH wants to tell his father of his disappointment and why. If your DH still doesn't want to take it (nicely) any further, then you should respect your husband's wishes on this matter, although you might want to just mention to your husband that there is a chance that after his father dies your DH might start to dwell on it more, and realise that actually he does want to understand what is going on in his father's brain, but by then it will be far too late to ask him.
EE1980, you sound afraid that your husband will verbally abuse you, and maybe even divorce you if you do ask the questions that your FIL has actually asked you to ask him. If this is the case OP, I think that it might be the right time for you to divorce your DH? This is no way to live OP, no way at all. Good luck x