Multilingual Children

Hello all, its great to read these discussions.  I wanted to share my situation with expert parents to get a little advice on raising my child. 
We come from a Spanish speaking country and my son was raised in Spanish during his first two years (at home and at school). I think his language skills are good because he was saying his first words at age 1 (words we could understand like cookie, eat, milk, car). At the beginning of this year we moved to Hong Kong and I registered him at a local bilingual school (he has classes both in English and Cantonese), and he learned English really fast (I can say his English level is as good as Spanish), and he is now understanding lots of Cantonese and speaks some words with local kids or at public places. However this year the school added Mandarin lessons to the curricula and I am a little concerned if this is maybe too much for him. I can help him at home with English and Mandarin, reviewing homework and practicing new vocabulary. However I dont know if it is already too much for him and dont want to put him under too much pressure. He seems happy to go to school every day and does not show any sign of stress. I have talk to his professor at school and explained her that I don’t want to put much pressure on him on memorizing characters and learning vocabulary (local parents are crazy about exams, grades and memorizing words and characters), and because he is just in K1 I think he will eventually catch up with the languages. But I think we need some guidance to be sure he does well and does not feel stress about this language confusion.