I'm a Chaos Marines player.
Got to find the time to post some pics.
Wish we could PBEM!
I actually have suggestion.
I used a nice way to build up armies for my brothers without having to spend a dime.
Since you must have the basic rulebook, you should have all the soldiers rules for your army and some of the enemies.
Thus you have all you need except miniatures.
So, you get head down into the rulebooks and build on paper an Army you think it's good and you use a small stone or coins or empty base to keep track of where your guys are and try it out.
This way you can see how that army plays out without having to buy hundreds of soldiers or vehicles beforehand.
When you think you nailed down an army of your liking you start buying in small caches.
To avoid getting bored doing too many of the same soldiers, break down the numbers and buy a bit of stuff for each selection.
I mean, if you have to buy 40 space marines buy 10 of them and paint them, then get that tank you wanted and paint it. Get 10 more Space Marines and then those 5 terminator you wanted and so on.
Doing too many of the same will easily bore your son out.
Hope to have made any sense.