LJC, I am very sorry that you’ve had this terrible shock. Making plans, affection at the party......then gone.
You do need much support, but not from the person who has suddenly thrown a grenade into your life.
If he was upset about your parents’ presence, he would have discussed that, and it wouldn’t have affected his love for you. There is likely an OW, and she may have recently issued an ultimatum, hence his hasty move.
You want to seek couples counseling for closure, but if he has been cheating, he will likely lie to you and the counselor, thereby sabotaging the process.
You, however, need to have the support of individual counseling to express your feelings and organize your thoughts, as you gain your equilibrium and make decisions for you and your child.