Yup, new battle pass today. Heard all about it from DS11 so asked him for you.
It's a series of levels they can access to earn things and gain achievements. If he doesn't get it , he won't be able to "level" up as rapidly as his friends who have it. DS11 says you can still play, you'll just be slower to improve unless you buy stuff in the shop, and he says you still keep all your stuff. I have now reminded him about the pass, and he's off to see if it's released yet.
No pop up ads on our Xbox, have you set the parental controls on the console? If your DS is using the console to watch videos of others playing Fortnite on Youtube ( yes that's a thing) that's where the pop-up ads appear, and the inappropriate ones can be binned by setting YouTube restrictions.
And Fortnite is not a evil game, provided you educate yourself about it, educate your son about online stranger danger, and supervise.
Don't ban him from it, control the amount of time that he spends on it. DS11 is limited to one hour after school, and 3-4 hours on weekends.He thinks this is shit, but has no choice in the matter.
To be honest, buying him a battle pass for him to earn his extra's in game rather than want to buy them in the shop may actually be a better option.Less nagging for money.