can´t play online, but i have already exchange an xbox live gold card

How long was the membership? If it was a trial membership then you are only granted limited access to Live services. That access is usually limited to the game it game with and is not valid for online ranked matches.

If it was a full membership then it can take up to 48 hours to fully apply to your account.

You can check your membership status by logging in to account.microsoft.com and clicking on services and subscriptions.

Do you mean redeemed a Gold membership? What is the error you are seeing when you try to play online?

If you've recently entered your membership code, try restarting your xbox. Do this by pressing the xbox button>press settings>press restart.

"Xbox Live Gold" card is for buying the subscription for Xbox Live Gold, which your son will already have as he's able to play Fortnite online.

Xbox and Xbox Live cards should be same, but to be sure just buy the Xbox Gift card. That will let him buy the V-bucks.

You need to have an Xbox Live Gold Subscription to use any Xbox game online. This includes playing the game online or doing anything that requires access to the server. You can pick a 1 month card for $10 at your nearest store or a 1 year card for $60 (50%
off compared to 12 1 month cards).

Sometimes they have Xbox Live Gold weekends which allow anyone without a subscription to play online for free and some games come with a trial Gold subscription

If you already have Xbox Live Gold, then it is possible that the servers for the game are temporarily down and you would need to check EA;s forums to find out..

Gold can only be purchased via credit card/Paypal or by purchasing a prepaid gold card in a store. If you've already redeemed the gift card, it cannot be exchanged for a gold card.