The bulk bowden ptfe tube is what you need. I bought it from Amazon for my Creator Pro. It seems the ones I purchased precut were a little short so this my best solution.

https://www.amazon.com/gp/product/B01CUPV90M/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1

Hello! I bought my first Ender 3 pro about 8 weeks ago, and my second E3P 6 weeks ago. Some thoughts for your post:

Its a very easy printer, that is *almost* build and go. For assembly I followed this videoand I credit that with being up and running within two hours of opening the box. A lot of people (and I feel like this sub esp) will shit on this machine but for someone just getting into the hobby its great. Its going to teach the best parts of the hobby, like finished prints and making cool shit and the absolute worst parts, like when something goes horribly wrong. To answer some more of your questions:

I think that stand will work, but I might switch out the two non locking wheels for locking wheels. Or cable latch it (like a gas grill) to a wall. I have my two prints (Jake and Elwood and yes I named them) on a dresser in my room and they do some shaking, as it the nature with a bed slinger design. But I dig the cart.

I started with PLA+, eSun that I bought off amazon. I've had very little filament issues and its produced some great prints. From my research Hatchback is great as well. Stick with PLA or PLA+ for now.

The E3P comes with a filament holder and for the purpose of the cart, that's perfect for it. There are some better designed holders out there, but they would take up more space on the side but I would encourage you to encourage your son to look at it. Problem solving like that is what makes this so much fun.

Other tools? Here's a quick list of stuff I'd recommended having on hand when starting:

New metal extruder, bed springs and knobs. Metal extruder works much much better and is more reliible, bed springs are more beefy and will help keep it level longer.

Media card reader. Amazing in my mid 30s I had no way of reading a microSD card easily between my Macbook Pro and Desktop. This made things easier. But if you have a way of reading media cards already skip ahead!

Some Bowden tube! Its nice to have on hand as you don't want to get jammed up because of a, well, jammed oh my god my cat just scratched the shit out of my leg

Glue sticks! Some people will say you don't need them, other people swear by them, and honestly they are both right. Using them to help the PLA stick down is a massive help espially with the build surface is being fussy. In theory you shouldn't need it, but it helps when your learning the ropes. Note: I'm a fully functional adult male and the tops of these things are a fucking nightmare to get off

Some nice flush wire cutters, great for cleaning up prints or cutting supports out or trimming PLA before feeding it into the machine. The ones the machine come with are OK, these are 100x nicer. Warning that are indeed sharp

-Some ziplock bags to keep opened but not currently use PLA in. I use One Gallon freezer bags myself. Each roll has a silicone pouch with it to keep moisture out, I toss that in the bag as well.

I'd have 2 or 3 rolls of PLA on hand, that is a good amount to start with. Amazon also has sampler packs of PLA that come with different colors, and its 200g instead of the 1kg for around the same price. The pro comes with a small roll to get you started but its not the best. Pick some colors or maybe even two base colors and something wild. I just got my first rainbow roll and its been pretty fun to print with.

Sorry for the long post. If you have any questions or need help, I'd head over to the Ender3 subreddit and ask on here as well.
Edited a few words and now gotta go clean up my leg