A salaried employee will have a fixed annual salary which will be paid each year regardless of number of hours worked. They are not paid by the hour.
A non-salaried employee will be on an hourly wage but won't have a fixed annual salary. They may work different hours each week and will be paid according to number of hours worked.
If he is salaried (fixed annual salary), he can't meet the requirements unless his employer can guarantee minimum annual overtime earnings of £2,480 every single year (to make it up to £18,600 per year).
If he is non-salaried (paid by the hour), then they will take the average total amount earned over the last 6 months and use it to work out his annual salary.
In this case, you add up ALL his earnings over the 6 months (including overtime), divide by 6 and multiply by 12. If this amount is at least £18,600, you meet the requirements.
If he is non-salaried, the employer letter should state this and only give the hourly wage (plus maybe also the total amount earned over the last 6 months), not an annual salary.