ESF kinder one more question

Dear all,
Thank you for the information!
I'd like to ask one more thing: I'd say my son's English standard is fair. He can understand and follow the instructions but he is weak in speaking. I'm now considering to let him join the ESF K1 now. (not satisfy about the kinder he is now studying, want to change to IS) Will he be difficult to adapt and will it be too late to brush up his English (I aim at the ESF/PIS/other IS