S.B.
I also live in California and have a great deal of experience with the "Disneyland Dad" thing.
My ex took me to court for years trying to get custody of our son. He didn't really want custody, he just wanted to hurt me and get out of child support.
In my county, parents are required to attend mediation prior to any modifications taking place. Your son is old enough to meet with the mediator.
My ex husband thought he was being slick by bragging about how good my son had it with him, all the trips they took, how he had basically anything he wanted and had expressed a clear desire to live with his father.
The mediator just looked at him and said, "Yeah, well, it doesn't work like that. Some kids want to live with their dad just because he has a bigger TV.
It doesn't mean that's what's best for them".
My ex spent a lot of years and a lot of money on attorneys, etc.
He never got custody of our son.
If you are not required to go to mediation in your county, I suggest requesting it. Mediation is where the parents try to come to an agreement and can express their concerns. You have a right to your concerns.
You also have to keep in mind that lots of kids want lots of things, but they can be fickle and self-centered little creatures. They just want what they want and half the time, they don't even reallly KNOW what they want, let alone what best for them.
I obviously don't know your entire situation, but there doesn't seem to be any real valid reason for a change. He's in one spot for the school week, has every weekend and shares summers with his dad. That's pretty typical.
I know this is scary, but hang in there. Request mediation, don't agree to anything yet. Make sure you respond to the petition in a timely manner. You might also want to suggest family counseling to help your son deal with the two family situation as he enters puberty because it can be rough enough on kids in the first place. I would want to work on heading off any acting out before it starts. That will demonstrate proactive parenting.
Best wishes.