One way to tell is slowly draw your son's bow back,and have some one stand behind you and either watch, or lightly touch your shoulder blades. There should be noticeable tension as your shoulder blades are squeezing inwards towards each other.
With some archers, they either overuse triceps on the draw arm, or do not fully push to tension on the bow arm. It will be distracting for you, but just concentrate on doing things how you do it.
A basic way to get a feel without drawing a bow is to stand with fists to edge of chest, elbows out (ie-chicken dance position) and have someone push against your elbows forward while you try to push your elbows behind you. This will engage the muscles you need for drawing a bow.
Also remember that creeping your neck forward to anchor will also shorten draw length. Ideally, find a local coach, and spend an hour or two getting some 1-1 instruction.
Just have fun, and enjoy shooting. It really is what it is all about.