I pack heavy by FT standards and even I think that is way too much stuff. We managed a 17 day trip across Europe with a toddler with major food allergies with only 1 checked bag and a small carry-on per person. I'm pretty sure you can handle SF with the same amount.

Honestly, the #1 thing to cut down on is probably baby gear. Unless your child is medically fragile, all you need to take is a car seat and stroller. Your stepson can borrow or rent a port-a-crib or whatever your child needs to sleep in, same with a highchair. The little one doesn't need a full arsenal of toys; a few favorites will do.

Also, I think your stepson is mistaken about gate checking bags unless all the overhead bins are already full when you board and you are forced to gate check.