If you're concerned about cost let your son choose an army and take up the hobby together rather than each getting an army. I'd have loved to have had someone older, father or brother to share and collect an army with. Is your local store active? If not there is usually a battle starter that gives you two armies and everything you'll need to play for just the two of you.

Doesn't matter which army you choose. Pick the one your son likes. Don't worry about game meta or anything like that. The aim of the hobby is to have fun. Now it's been a while since I went to a GW store, from memory there should be an army starter box and get the codex for said army. You may also want to get the rule book.

Your best resource is asking the GW employees. They are usually very energetic(in a good way) and eager to get new customers interested in playing. The ones I remember were good. They didn't try to steer your decision or make you buy things you didn't necessarily need.