Hi

My son is 16 years old ,17 next August, 5'10"

He wants to start doing trials events. What size engine is he allowed?

According to the ACU standing regulations, as a 16 year old now he can elect to ride either as a Youth A class rider or as an adult on an event by event basis until the end of 2019. As an adult he would have no engine size restriction, as a Youth A class rider he would be restricted to 125cc monoshock or 250cc twinshock machines. 

Of course, this applies to events operating under ACU jurisdiction only.