M.S.
I'd spend more time with the 10-year-old, give him lots of love and positive attention. It appears by his actions he is in need of both. Try to redirect his behavior and energy (for example, tell him how fun it would be to stack the chairs in sets of two and then see who can crawl through the sets the fastest... or set the chairs in a circle and play musical chairs). Burn off his energy, need for attention and save everyones frustration. The child is probably a product of a mother unable to set limits and boundaries. Are you able to change her? Doubt it, somethings are left better in the hands of professionals and/or family. Can you change the atmosphere when her son is around you and your children, perhaps ...