No personal experience, other than as a child myself - going from a small school I'd been in since nursery, to a huge school in another country. Its basically really scary to start with - a whole different scale of 'world' to live in. I was only there a few months (my father was on sabbatical) so never got totally used to it, but I definitely felt happier after the first three weeks or so...I'd give it longer than two weeks. I do remember liking the teacher, and having some friends, but the size of the place, the number of kids, and the different rules made me feel simply unsafe - and I wasn't used to feeling that way.
If it was dd1 going through it, I think I'd use a mix of chats, role plays (I use her cuddlies, or small dolls that she loves to play with - not sure what I'd do with a ds!), helping her find small things/places/moments in the new school which feel safe or happy to focus on, etc.
It must be heartbreaking to see him sad, but I would try to put your self-doubt about your choice of school to one side for a bit longer, and focus on helping your ds through his sadness. Its very normal for him to be missing an old, safe, cosy environment (grieving, even) and for that matter, it would take me, as an adult, more than two weeks to feel safe and secure in a big new workplace! Wishing him happier soon...