I think you first have to establish what area of the music "business" he wants to break into. If he doesn't play an instrument or write his own songs then at the moment it's just singing along (I know, he's only 12!). So another vote for YouTube. Look at other YouTubers who are doing the same thing, you'll probably find that they are marketing themselves as widely as possible, using Instagram, Twitter anf FB also. Often the more successful channels use better camera work and editing, so it is worth getting him involved in this too. Post production is often overlooked and really makes a difference between someone singing into a hairbrush in their bedroom, and someone creating good quality watchable videos.
I'd also encourage him to learn an instrument, or at the very least how to create digital music so he can write songs. This is not so important if he just wants to upload videos, but if he wants to be a serious musician then an ear for music is a must.
I'd also stress that performing in front of people is a skill that can only be mastered with practice. I'd take every opportunity around - local talent shows, school performance evenings, open mic nights etc. You see it a lot on talent shows where people are good at singing in their own safe space, but put them on stage and they are like a rabbit in headlights. I played and sang in a band from 14, it took me at least half a dozen performances to turn my nervous energy into an enhancement tool rather than a restraining device.
I guess what I'm saying is, if he wants to be a serious musician then he needs to get out there. If he wants to be a singing YouTube star then he needs to produce something extra special that no-one else has, and market himself to be clicked on in preference to the other hundreds of thousands of wannabess on the net. Above all, just encourage him. His enjoyment of singing should be paramount
