I don't have this issue with my hubby. We ALWAYS go to his mother's (grrrrr). But I do have something to say! (If you knew me, you would NOT be surprised!!)
My mother didn't have a grand childhood. There are parts she remembers fondly and parts not so fondly. There are times during the year that she gets down or wants to be left alone. There are times when whatever holiday we are on remind her of her childhood and make her blue, or upset. It could be the same with your husband.
You say your husband is your best friend - so, ask him about it. Just ask if Christmas is hard for him due to his childhood. That's a starting point. From there, you can maybe talk about what you can do as a couple to help him deal with his Christmas issues.
As for the in-law thing, just watch to make sure he is included, but not forced. My husband literally sat on the sidelines for like 5 years. We've been married for 14 now and he finally gets just a little bit involved.
Those are what I suggest. Sorry that it's all I've got. Good luck!