He asked a question, said he was young, and asked for help. I'm telling him the truth... there is no better controller for Smash than the gamecube controller. If he didn't want people to dictate their opinions... he wouldn't ask. And since you're considering not using a gamecube controller, I give you the same feedback... GCN controller or bust. There is no reason in not starting your training with proper muscle memory on a proper controller. I can play on 3ds I can play on the gamepad, your ceiling will be limited it's not personal opinion, the gamecube controller is objectively better for the game- it's not required, it's just better and if he's young he should start ASAP and just get over the small learning curve.
If he didn't care that much about his Smash, why would he ask?
I hope he has fun with the game at whatever level, but I'm strongly suggesting you get over the hump and pick up a gamecube controller when they come out... you will thank me later.
You're wrong btw. Wii U Pro was superior in every objective metric but two, input delay (being wireless, it had about 5-10 ms more delay or half a frame, which would translate to 1 frame slower inputs 40% of the time) and octagonal edges around the left control stick.
Switch Pro is also objectively better than GCC in every objective metric except for the octagonal rigs, with input delay still needing to be tested to correctly measure which is better. I tested input delay on Pokken and Switch Pro has actually less input delay when played wired than the GCC though. But some of the pro players that played at the invitational were complaining about input lag in Smash (which I didn't notice myself when playing the demo, but I don't want to go by "feeling", once I get the game I will objectively measure which one has more input delay).
To be thorough:
1-) Stick placement, button size/placement, stick "smoothess", controller shape, controller size is all subjective.
2-) Digital Shoulders - Pro controllers objectively better (faster input).
3-) Face Button Responsiveness (Clickiness) - Pro controllers objectively better (faster inputs).
4-) Left Stick Sensitivity/Amount of Inputs - Pro controllers objectively better (Give more room for different angles. It's easier to walk. It's easier to platform drop. Easier to SDI).
5-) Number of buttons - Pro controllers objectively better (allowing for you to customize your controls in ways that allows for stuff that would be literally impossible in the GCC)
6-) Number of buttons that can be pressed at the same time (X + Y, A + B) - Pro controllers objectively better (With the new macros present in Smash Ultimate, someone who customizes their controls could get a ton of use of the position of the face buttons).
7-) Octagonal Ridges - GCC objectively better (allows for easier time inputting precise angles.)
8-) Input Delay - Still needs to be tested in Smash
The problem with "conventional wisdom" is that once it is not true anymore all of the people who never actually "analyzed" why that conventional wisdom was actually true, will keep on believing this conventional wisdom forever. And because conventional wisdom is literally what the majority believes to be true, this conventional wisdom will remain "real" for as long as people don't care to actually think for themselves.
There's a reason most people need to "claw" in Melee or hold the controller with the middle finger on the shield instead of the index. Yet with the Wii U Pro (and the Switch Pro) you could do all those things that require clawing or middle finger holding with a regular, comfortable hold and even (if you customize your controls) do it without moving a single finger off a button. Can you imagine that? You can perform every input in the game without moving a single finger, instead of having to screw up your hand immensely doing a lot of wrist/finger movements in order to get all the inputs you need? Yea, GCC is the "best" controller for sure...
At the end of the day, what will kill or make this controller is input delay. I used a GCC in Smash 4 because I didn't want a 1 frame input delay, there were just some scenarios where I needed instant reactions (ledge traps, ledge trumps and whiff punishes mostly). If the Switch Pro proves to have less input delay though, there would be 0 reason for me not to use it. I might make my own octagonal ridge for it too.