It's thought that at least 10g of gluten per day is needed.
White bread - 2-3 g per slice
Weetabix/Shredded wheat - 2g each
Serving pasta - 6g
If you've been off gluten it will take a while for the body to develop antibodies to it (which is what the test measures).
I think you can buy gluten powder to add to foods if your child refuses to eat things which they know cause problems.
I think if you're going down the route of testing it's worth making sure adequate gluten has been consumed. Make sure the GP sees him before/after - so they can see the difference in him on/off gluten.