im trying to figure out which piece I need to order how do I know?

Hi Eva,

@andrewsawesome has given you the information you need; I thought I'd expand on that a bit for your edification.

Apple's phone screens generally consist of one, two or three parts; the backlight (on LCD screens like yours, not on newer OLED screens), the LCD/OLED display itself, and the digitizer.

LCD screens don't generate light so they require a backlight, which is simply a square of material that lights up behind the LCD, which variously lets different colors of light through so you can see it. On the OLED screens the screen itself generates the light; individual pixels glow on their own so no backlight is needed.

Generally on top of that is a clear piece of glass with invisible circuitry embedded in it that detects the presence of a finger. The very latest screens combine the OLED layer with the digitizer so now there's only one layer.

Anyway, no matter which one it is, Apple bonds the layers together. Although it is possible to separate them, it requires specialized tools and expertise to do so without breaking something. It's not something your average do-it-yourselfer can successfully tackle in most cases.

The point here is that it doesn't matter what part of the display has gone bad; the backlight, the digitizer or the LCD; they only come as a single assembly so that's what needs to be replaced. In your case the LCD has cracked or gone bad; on other phones it may be that the digitizer glass has broken or quit working and in very rare cases the backlight may fail. But whatever it is, the solution is to replace the screen as is the case with your son's phone.

I don't know if you're planning on replacing the screen yourself, but since that's what this site is all about, I'm hoping you are. A couple of notes that might help:

First of all, the home button is paired to the motherboard in the phone, so you will have to transfer it to the new screen if you want the Touch ID to keep working. The guide Andrew linked to includes instructions on that.

Second, replacing the screen will disable the True Tone functionality of the screen. It can be restored by hooking the old screen up to a chip programmer and reading an identification number from it, then programming that number into the replacement screen. It's easy to do, but you have to have the programmer; I just ordered one for myself for $85 USD. The phone will work fine without True Tone, but you might be able to find a local repair shop that can copy the data for you; they shouldn't charge much for that service as it should take them about 5 minute for the entire operation. You can always save the old screen for some date in the future when you decide to get that data transferred to the new screen.

Anyway, you'll want to buy a replacement screen; I've had good luck with aftermarket screens in most cases, so it's your choice as to whether you want a genuine Apple screen or a much cheaper aftermarket one. You can buy it here and support iFixit's repair efforts, or you can go shopping on places like Amazon, eBay and AliExpress to save money.

In order to reseal the phone against water, the screen adhesive will need to be replaced as well; I'd suggest buying more than one; they're cheap but it's easy to mess it up so it's a good idea to have a spare on hand, plus you never know if you may need to open the phone up again for some reason.
That's all I can think of at the moment, so I'll just wish you luck on your repair. Let us know how it goes.