For the safety of using aromatherapy oils in dilutions safe enough for the very young ones, make sure your Aromatherapist is accredited and insured and knowledgeable about the essential oils, not just someone who has done a short course like a few evening classes and practises it as a 'beauty therapy' and calls themselves an aromatherapist. There is a huge difference.
I've found the folks at Findhorn very helpful.
I'm rurally based and not near enough for individual therapists' visits etc. apart from at an eye-wateringly high price, so over the last 20+years, I've learned to do a lot myself. I always checked Valerie Worwood's 'Fragrant Pharmacy' chapters on uses of aromatherapy oils for babies and children. Some IIRC were less than 1 drop of EO to 10ml carrier, (so needed to be made in a larger bottle) so do take a lot of care.
One rule of thumb in the absence of consensus for things like fish oil or no dosage given by the manufacturer is to use bodyweight. How much is an adult dose, how much does the average adult that's aimed at weigh? Then scale the dose by bodyweight.
For herb teas, adults are 1 cup 3 times per day, children are a third of a cup 3 times a day, but alter that on how big the child is.
"Children" as in under 5 years old appear to have been removed from a lot of herbal and other medicine labels, again, use a reputable practitioner, for example NIMH www.nimh.org.uk/
Always remember to do a test patch of anything you may be thinking of using on skin, apply it and leave for 48 hours.
I'm a big fan of using essences, either Bach, Bush Flower or Findhorn for preference and feel that they pose minimal threat to a child if the dosage is accidentally given wrongly as opposed to herbs or EO.
It's always difficult to single out what makes a difference to a child, mainly because we don't try a single thing at a time, test it, evaluate it and then move on to something else. All you need to do is ensure you're not causing any harm. Good luck with your quest, I hope you can find some things, or combinations of things to help your son, and that you can let other parents know what's worked for you 