The most important thing to have to become a "sparky" is Common sense, patience, a logical mind and a willingness to learn, I know they say about qualifications, but that is on paper, he should go and talk to local companies, I left school without even taking the then GCEs, but got an apprenticeship and have been sparkying for over 40 years, it is a good trade with lots of laughs and the potential to earn good money, just tell him lo listen and learn, wish him good luck, Ray
If he's interested in an apprentiship, with local council say, they normally ask for 4/5 GCSEs inc Maths - then they have to pass a basic skills test (Maths, English, etc) as part of interview process.
Following on from the previous answers - your son would need at least 5 grade A-C including Maths and English as a minimum requirement. Equally important is looking for an employer - most colleges require young people to have an employer placement before you can study on an apprenticeship. He could get some work experience with an established electrician/electrical company which would certainly help. Hope this is useful.