I honestly can't believe so many responders missed the real question here.
Your problem isn't that your son needs to learn to ski.
Your problem is that your brother and your sons other uncles let your son down when he needed them the most.
It wasn't just the year your husband died, as if maybe they thought he just needed some time to grieve. It was every year since then. Pretty much since he was the age to be able to participate.
Your brother may have left your son out the first year because he was uncomfortable with it so close to your husband's death but to exclude him for the next 4 years as well?
It sounds to me like your brother's wife nagged him enough that he finally invited your son and he keeps calling, hoping that your son will think it's going to be too hard and back out. Maybe he's afraid he'll have to spend the whole time with your son, teaching him and taking care of him.
Does he realize your son is 17?
I'm guessing that they're not going to the mountains to do some extreme skiing. That they'll be at a resort or something that offers lessons and has bunny hills and beginner trails.
I'm not sure what your brother thinks your son would be "getting into" other than learning to ski and having to spend the first days on the beginner trails...isn't that what you're supposed to do when you first learn to ski? Others have mentioned that he would indeed be skiing alone anyway so he wouldn't be holding them back.
Tell your brother that you didn't say anything about him excluding your son for the last 5 years because you didn't want to create a problem in the family... he is your brother after all. You are speaking up now because his constant negative attitude about this trip is unacceptable.
Remind him that if he didn't want your son to go along then he shouldn't have invited him. Now that he has, he needs back off and realize that your son is 17 and is more than capable of learning to ski without a babysitter
It sounds to me like your brother has some issues of his own to work out, what with still acting like a child well into adulthood.