as your son progresses you might consider even just brushing on primer. I like Vallejo stuff and they have many different colours to suit your projects.
When I first started I brushed on primer usually Grey; as mentioned earlier, the darker the colours make red, white, yellow muted and difficult to cover up. just take some primer up onto the brush and dab away to ensure that there are no bubbles. Let dry at least 15-20 mins between coats.
I brushed on primer for a few years and then most recently switched to an airbrush. Today, I like starting with black or Panzer grey primer spray down the entire model, Then using a white primer spray Zenithal highlight the miniature from the “source” of lighting above the mini. Particularily focusing on the facial area. That area is where most attention is going to be focused when complete. It’s subtle but makes a huge difference.