Yes. Your SO should go to his son's wedding. The son is not boycotting you...he is thinking of his momma.
Ginny, are a big person, and I must say, I have a great deal of respect for you. Your SO must appreciate you so much!
If your SO doesn't go to the wedding, his son (and the entire family on that side) will blame YOU. Communicate this to your SO.
Tell your SO that it is best FOR YOU and FOR HIM and FOR THE SON if he goes to the wedding.
Someday, it will come back to bless you both.
And what a delicious day of yarn shopping you should have on the day of that wedding!