No, IMHO attending is no sin at all, let alone not a mortal sin.
I have had some SERIOUS disagreements over wedding issues with others on this board.
Some observations:
The"situation" is not ideal but he’s your son. Consider what will happen if you DON’T attend:
a. Your non-appearance will fuel flames of anti-Catholic stereotypes of Catholics as sin-obsessed or hardhearted.
b. Son presumably only gets 1 wedding…if you miss it, you may be severing a family tie irreparably. Good luck seeing any grandkids…ever.
Contrast this with the dogmatic and IMHO overly legalistic kneejerk reaction some folks have of, “the marriage is invalid so I would refuse to attend!” IMHO, such a reaction is…dogmatic and overly legalistic (to say nothing of potentially wrong vis-a-vis the invalidity).
Some will quote the Catechism. I’d prefer the Beatitudes. Or just, “love your neighbor.” Does not going make you a peacemaker? Does not going = love? IMHO, NO. It makes you…overly legalistic.
Others will counter with some permutation of, “if you take a stand, and not go, you will be respected.” I respond, “don’t act like a jerk and expected to be treated as a non-jerk.” IMHO, refusing to attend your own son’s wedding falls into the category of “being a jerk.” Moreover, it is a major, life-changing decision which has serious negative consequences and cannot be undone.
So I vote “go.” And feel NO guilt.