Chiming in to add celiac disease to sanne's list of conditions that can cause ADHD-like symptoms; if he has any symptoms and/or if autoimmune conditions run in your family, consider asking for the screening blood test.
https://celiac.org/celiac-disease/resources/checklist/
https://www.verywell.com/gluten-and-adhd-562627
https://www.ncbi.nlm.nih.gov/pubmed/21977364