I had similar experience, my son was in international school for N1 & K1 then I transferred to SKH (Central) in K2.
Honestly, he had a difficult time understanding what is going on around him. It's cultural shock to him! (He was still playing sand in ESF back in K1) Luckily the K2 teachers are very understanding and there's not much homework.
Please be prepared that K3 teachers are more like Primary 1 teachers who will help you train your kid to behave like a primary student (writing homework record book in Chinese) and the teaching method I suspect is more traditional than Victoria. There will be "revision" (ie. dictation) for English and Chinese every week. 2-3 homework everyday.
You need to think where you want you kid to go to in Primary school to make the switch.
And there's no guarantee that SKH will help you get into SPCC. You need to train your son personally at home to be fluent in Chinese to do that (not sure about PTH though)