He's 12 I'd say he's adult enough (imo) to learn submissions. In time though.
For me the ground game and teaching it seemed to come naturally, just start with simple effective basic technique, teach it detailed as possible. Explain the move while doing it on him (or if you have a training partner who can help you show the flow drill).
Say pass the guard explaining while whichever pass method you are best at in detail. Get to side control have him do the bridge and shrimp back into guard technique, then have him scissor sweep, as you are swept be sure to block the knee as he comes up (will keep you from getting mounted, and put him in side control) then you reclaim guard by bridging and shrimping, have him pass, reclaim guard by bridging up and shrimping again then scissor sweep, have him block the knee putting you in side control. Then just simply continue running through this same drill several times in a good flow 'til it comes natural. Once both of you can do this easily, at a set pace, without stopping to figure things out you can progress to adding things in such as submissions.
I like to do this, explaining every detail in depth for ATLEAST 30mins when teaching basics. Explain passing the guard, how you do it, every little detail, every little detail of side control and getting your weight on the person good, every detail of the sidecontrol escape, ect.. If you are familiar with the three common different levels, as I like to call them, of side control once he has the initial flow drill down you can show him other types of side control and the escapes. Once you have instructing the first flow drill down pact (I find it helps to take notes as I show basics sometimes) you might find it easy to develop your own flow drills with the basics or do some of the others people have posted here. Making variations, ect..
The one I posted has just always been one of my favorites, I find it very effective in making a person very good at reclaiming guard and keeping a person from passing. Keep in mine when I type I'm generally speaking no-gi, however it works the same with the gi.
If some things were hard to understand I apologize, in something of a hurry.
Merry Christmas and Happy Holidays.
