The most useful tests for us have been organic acid test, hair metals test, urine porphyrins and lipid peroxidation oxidative damage marker. We monitor and test these periodically. Also tested positive for KPU and retesting that. Haven't done any specialized immune / antibodies / titer testing.

You should consult your pediatrician and probably do a genetic test to rule out any known conditions which might help you get on the right track for treatment.

I would test for Lyme.. Our doctor believes that most ASD is congenital Lyme disease: https://www.youtube.com/watch?v=-z3kRDYcvhA
Out of the gate you could do an OAT (great plains lab), check Vit D, ferritin, IgG,IgA, IgenX IgG IgM

We started with the list in Dr. Kenneth Bock's "Healing the New Childhood Epidemics". Most came back normal but we did discover that DD's blood carnitine level was low and supplementing with acetyl-L-carnitine lead to a dramatic jump in functioning.

We tried doing the blood work as indicated in one of Jenny McCarthy's books. It was interesting to see. In a different book I read that doing those tests can be pointless sometimes because things change with everything the kid eats. And it becomes expensive.