Son was banned for 18 months and done drink awareness course, do we need to tell insurance company?

Yes, you need to tell the insurance company. I am in the same situation as your son, I''m about to get my licence back after having a ban also. Your son will not be covered by the insurance company in the case of having a car accident unless it has been declared to the insurance company about his ban. Insurance is going to be really expensive though as drink driving is a big no no for insurance companies!!

When you got the 25% taken off your ban, was it exactly calculated or was there some leeway? eg did you get your licence back before the exact 25% had been taken off? thanks