Hi,
Ive owned a scooter - you need a provisional driving licence and a bike for the test (i had a 50cc, not sure what the max allowance is though for learners). You dont need to complete a theory test, however he will need to know the rules of the road etc as he will be questioned about it during the test.
Look in your yellow pages for CBT trainers, it cost me about £50, its a full day type thing where you learn the basics of riding a bike and then a group of you go out on the road for your 'test' the instructor can talk to you via a special helmet so you know where you are going and what to do etc... you can call them and ask for the details about the size engine etc :)
If you do ok and dont endanger anyone you get your CBT certificate which is valid for 2 years. This entitles you to ride your scooter/moped on a learners driving licence whilst displaying 'L' Plates.
hth,
Roxanne