I cut my son's hair myself. I have paid the $15 + tip, and come home every time dissatisfied, and ended up re-cutting it myself.
We use the regular store bought clippers with guards. In the summer, we go really short, just shy of a total buzz cut.
If you want a place to start, what I do is start with the top, and a #4 guard (I think they are pretty standardly numbered for length.) Then, I do a #2 on the back and sides. Then, we blend with a #3. You could try that first, and see if it seems to be ok. You can always go a little shorter if this isn't short enough for you, but it also leaves a tiny bit of room for fixing, if you don't like the way it turns out.
Definitely - do it yourself. Little boys are so easy, and I agree with the short cut for summer. Too hot, too sweaty for long locks!