your son will probably go into yr 9. The cut off really only is applicable to children starting school for the first time to ensure they get long enough in their first year at school and the rest follows on from there. Some kids are 12 when they start yr 8, some will be 13 already. Likely your school will look at what your child's experience is and place him in yr 9.
I reckon this is spot on - I have a 12 year old who will be 13 in May and he is currently in year 8. He is a little younger than his classmates, due to having started school in the UK at age 4 rather than 5 as they do here, so it sounds like your son is the same (but plus one year!)