sounds nice! come hell or high water before the mrs paint a miniature, but im eager to push jr into it. that said, if he is anything like me, he'll hate everything i like lol
Don't push. Wait until he's bored. Then just sit there and paint. Eventually he'll come and ask you what you're doing. That's when you carefully reel him in. Simply say "I'm painting" and show him one or two interesting minis and how cool they are. Make it look easy by saying things like "It's just like a coloring book, but not flat. You just have to colour inside the lines, but that's the hardest part". Then wait for his "Can I try". Don't ask him if he wants to try. The idea needs to mature in his own head. Make sure you have a few detailed minis ready, trimmed and primed (don't make him do the boring parts to begin with) that he can choose one from. Make sure there are plenty of brushes to use, cause he'll ruin a couple in the beginning. Then guide him through the simplest steps, bit by bit, so he gets time to digest each step. Blocking out the base colours. Then a wash (keep it simple), and maybe take before and after pictures so he can see what difference he's creating. Let him decide when each step is done, to some extent. Show him the bare minimums he needs to accomplish, but allow some leeway. Then thirdly show him some simple dry brushing or similar technique, and finally details. Explain that it's not necessary to paint the smallest details, like eyes, the first time since it's so hard.
Finally, it's better to wait until at least about the age of 10 or so. The nervous system isn't fully developed until about age 14, and younger children might have trouble with the motor skills. But that can differ from individual to individual.