Then Mom is basically going to dictate. She has status quo on her side, and your court order is too vague to argue.
You can try to obtain a literal 50/50 timeshare, but I wouldn't bet the bank on it.
You've also got to be realistic - it's not "a few months". It's actually 6 months and yes, the difference can be important. On top of that, your son is 13. While his wishes do not control, if he's dead against living with you the court might just take his opinion into consideration.

