I would encourage your stepson's parents to take him to see a board certified child psychiatrist (and have a full neurpsycholgical evaluation) or go to a Developmental Pediatrician for a true diagnosis of ADHD- hyper active type, combine type, or intatentitve type (the last formerly known as ADD many years ago.) Without a full evaluation, you really do not have a good diagnosis and psychologists can do evaluations that show consistency with the diagnosis, they can suspect the diagnosis, they can suggest that you go get a medical diagnosis, but they cannot diagnose ADHD of any type, because they are not medical doctors and they have no bussiness telling anyone about any drug or supplement.
Psychologists are valuable team members for treatment of properly diagnosed ADHD of all types.
Pediatricians can and do prescribe medication for ADHD, but this is not the very best source of medication knowledge for this specialty, nor are they appropriate diagnosticians, and they most certianly cannot provide you with the hours of diagnostic evaluation data that will give you a full treatment plan complete with educational needs and psycholgical and behavioral intervention strategy plans. Your step son deserves this kind of standard care.
Omega 3's are the new Acai berry! They are being pushed for everything right now and there is no harm in using them for good health, but do not expect any treatment value for ADHD (of any type.) Vyvanse is one of the newer ADHD medications and may have considerable value to your stepson, but so may a lot of other types of ADHD medication. How he responds to each one is individual, and it may take several trials to find the best response at the right dose, just like many other medications that are designed to releive troublesome medical symptoms from a whole slew of medical issues. Medication trials should be carried out by the best prescriber you can find, and that will most likely be a board certified child psychiatrist who specializes in these medications and issues and knows what to look for because they see it day in and day out.
That is my experience, get a full evaluation that takes many hours and includeds educational, psycholgical, speech and langauge, OT, neurolgical, etc, etc (according to his needs and symptoms) from either a neuropsycholgist or a Developmental pediatrician, and then get treatment based on that full evaluation from a Board Certified Child psychiatrist and follow his referal suggestions (for cognitive behaviroal therapy, speech, OT, educational interventions, social skills classes...) Let the very best prescriber who deals with these medications every day do the medical intervention. Make sure that medication is not your only treatment-he should have an evaluation that identifies all the therapy that is needed from soup to nuts.