i was totally against my kids going to school before they were 4-5 years of age. however, when i took our circumstances into account, i changed my mind. we live WAYYYY out in sai kung country park and although there are quite a few kids in our village, they all go to school.
my son was left at home with his little sister too much. he started at 1/2 day kindergarten (he was 3.5 but the youngest in his class was 2 years 9 months). he LOVED it!!! he went for 1/2 day, 5 days/week. he liked it so much (and we wanted him to speak more chinese) we put him into full day this year. we have not had one complaint from him.
my question to you is this: how much "playing" is he actually doing at home with grandma and helper? do they take him out to places like the science museum, ocean park, kowloon park, the supermarket, the playground? or do they sit at home in front of the tv all day? (we can't answer this, only you can.) if the answer is, yes, they take him out and about and do different things with him, then i'd say maybe he'll not be happier going to school if the answer is, he mostly sits in front of the tv and doesn't get a chance to interact with other kids, maybe he'd be happier going more often to "school". (even so, expect some crying for the first couple of weeks!)
keep in mind, pre-nursery is NOT and should not be sitting at a desk doing homework. they will be playing, a lot. just with different things, and in a different way. he'll be singing songs, reading stories, doing art work etc.
in the end, it all comes down to what will make YOUR child the happiest.