They reshuffle the class, means they change his teacher as well? or is still the same teacher dealing with him?

If is different teacher, probably he need time to adapt to the new teacher. As he may miss his previous teacher or friends who probably shuffle to another class.

Another possibility of him losing interest, could be the teacher fail to interest him. Some kids get interested during beginning of something new, but after some time they will see it as boring.

Since he already lost interest, putting him in for full day will stress him up. Try to create his interest to school. Ask him about his friends and find out if he have a good buddy. Get him some snack to bring to his buddy. The best way for kid to learn is by letting them be teacher. Try to make him feel that he wanted to teach you what he learn from the school today.

As for language barrier, it is not only the school responsibility. It is more on the parents/guardian responsibility to find ways to ease his learning of new language. If you find it awkward to suddenly speak mandarin to him. Get him a few educational DVD with singing and story telling in mandarin. Join him in watching that, and from there you can slowly talk mandarin as his secondary language with him. Do not rely on YouTube.. get a DVD, small kids learn better by repeating a DVD until they get very familiar. And the content of DVD is usually well planned by educationist.