18 mth probation means he is under the supervision of the probation service for that period, what he does with them is dependent on what orders he is/was given. He will have to go to meetings with them etc and co-operate with them. If he offends again whilst on the court order it is taken very seriously and he may have the order revoked and be resentenced for it.
BTW there is heavy pressure atm on sentencing parties NOT to send people to prison - its a government thing and influenced by the fact that the prisons are full to bursting....