I've got one at work.
I agree with the above, they've designed the network to be very resilient.
"Internet Outage" and "Grid Down" would need to be defined better.
Those are complex networks with a lot of redundancy, and it would be difficult to define how any individual outage would impact end users.
The public switched phone network is now mostly IP, even if you have an old analog phone at your house. They have pretty solid networks to support their systems, so usually issues are local, or 'statewide' type events. (yeah, sometimes better, but these get hard to define). With more end users relying on IP only based phone service via their internet provider, it gets even harder to define if the end point you are calling is going to be available.
But they are a good tool. An expensive tool, though. Think $1,500 for the phone, and $60 a month for the lowest tier service (mine has 10 minutes included per month).
You'd pay extra for a phone number out of the North American Numbering Plan, if not, you get something that looks more like an international phone number. With the international phone number (included in base price), the dialing is slightly more complex (011 + the full number you are calling)
Calling emergency services would require (ideally) using their 7 digit phone number to make sure you get to your local PSAP. Easy to load those in memory ahead of time, though.
Antenna -MUST- have a clear view of the sky, so no using this indoors unless you have an external antenna. It's not like a cell phone. You can't stick it in your pocked and expect it to ring when a call comes in.
It's a lot of money for a poorly defined event. A lot of people start talking about needing "comms when the grid goes down" without really understanding how these systems work and what the likeliness of it happening is. Is your brother really willing to spend that much money based on fear?
Better solutions out there now. Recently released cell phones have SMS through satellite capability. If his concerns are more local, get some good (not Chinese) GMRS radios. He's more likely to use those types of devices and be familiar with them than some expensive phone that sits in a closet waiting for some imagined zombie apocalypse.