Lost SIM card, how do I keep same number?

He needs to go to an ATT store for a replacement SIM card and have them activate it with his number. 

Thanks for your answer. I didn't mention it is a prepaid plan. Would they still help us at the ATT store?

Since you posted in the prepaid forum, I assumed it was a prepaid plan. Yes, they will still help you at the ATT store but you may need to find a corporate owned store. Authorized retail stores often won’t help prepaid customers.

Here's how your brother can go about getting a new SIM card, hello356!

As @sandblaster recommended, your brother can visit your nearest AT&T Store to pick up a replacement SIM.

It's also possible to obtain a new SIM card online or at an AT&T national retailer. Here's how to get started:

1. Find your SIM card size by going to the Device Support tutorial for your device and selecting Specifications.
2. Purchase a SIM card kit that is compatible with your device.
3. Remove the SIM card from the SIM kit and choose the right size.
4. Insert the SIM card.

You can find additional details about replacing a SIM or even activating an eSIM (if your device is eSIM capable) simply by viewing the details under DETAILED INFO at Get a replacement SIM.

We hope this info helps. Let us know if you have any other questions.

Thank you for reaching out to AT&T Community Forums.

Lar, AT&T Community Specialist

Thanks to both of you for your help! We will look into that.

We're glad that we could be of assistance, hello356!

Thank you for reaching out to AT&T Community Forums.

Let us know if you need any other support with AT&T products and services.

Enjoy the rest of your day.

Lar, AT&T Community Specialist