The average earnings for Jobseekers Benefit is worked out over the weeks that he was working, not over the entire year. So if he earned an average of over 300 Euro a week for the weeks that he worked, he will get full rate of Jobseekers Benefit. Even if he only worked one week in the year, its that week's pay that is used to calculate the average!!!!
If he opts for optional Jobseekers Allowance, it will be paid at the maximum rate of 144 Euro (unless he can show that one of the exceptions apply). A deduction may also be made because of parental income depending on your income.
Once his entitlement to Jobseekers Benefit ends after a year, the new lower rate will NOT be applied to him when he applies for his full (ie, not optional) Jobseekers Allowance, as once of the exceptions that applies is when you are switching over from Jobseekers Benefit to Jobseekers Allowance. He'll nearly be 25 at that point though, so it may not be of much benefit to him, as his rate will go to full rate once he turns 25.