Is the son obligated to address his maternal uncle's children as "my maternal uncle," even if they are younger than him, and not retaliate against their aggression towards him? And would the mother be sinful for preventing him from playing with them to reduce problems?

The child is not bound by Sharia (definition) rulings, but the guardian must discipline him and accustom him to obligations and to avoid prohibitions. It is not obligatory for the guardian to prevent the child from calling other children by their bare names or from retaliating against ill-treatment; in fact, negative consequences might arise from that. There is no sin upon you if you prevent your child from playing with your siblings to avoid problems, and you must use wisdom to combine filial piety with raising the child according to proper conduct.