British Curriculum vs IB K-12

My son currently studies at an IB school which they follow IB K-12 curriculum. I'm planning to move him to a British school.

However, there's one big concern regarding his age. He's going to grade 5 next school year which means he will have 8 more years, inclusive, until graduation from high school (until grade 12).

As his birthdate falls on November, in a British school he'll be assigned for Year 5 for the next school year which means he'll have 9 more years until he graduates high school (which is Year 13 in the British curriculum).

Does that mean my son will be one year behind if I move him to a British school?

Your response is highly appreciated!

On this particular topic, I would advise you to post on Mums in Bahrain.  It is a very active Facebook group.  They give good advice on topics such as these but very poor advice on visas/legalities etc :)

Many thanks XTang for the advice.