So your ex is joining you and your son on this trip? If so, I don't think you need anything different. I'd suggest making sure you bring any identification for your son, like his birth certificate, along with any custody agreement you have with your ex. Make sure you have valid passports and visas for yourself, your son and your ex if she is joining you.
If your ex is not going with you, you would also need a consent letter written by her, which may be needed in order for your son to travel with you. This should include the dates of travel, where you're going, and contact info for anyone going, including you.
Also, if unsure about what to do, you can reach out to the airport you intend to travel out of, and see if there is anything you need to do in order to bring your son along. You also should research the entry requirements to whichever country you plan to travel to. There may be other rules you need to follow when entering another country as well.