Dear Bozzy,
you could be describing my DS1, just after we moved. He arrived here when he was 2.5, was incredibly sociable and desperate to go to nursery. He hated it and the first year of school (they start at 3 here). All the changes and the language issues just proved too much. At school, he became a different child, solitary, aggressive and sad. Two years on, he is back to his old self. He has a best friend, likes people again, enjoys school and his three languages are coming on in leaps and bounds.
DS2, well, are you sitting comfortably? He has lots of single words, doesn't do conversation as such but does interact, with adults. A SALT came into the school, saw him for 10 minutes and then came back to do a full assessment with a developmental expert. Their report says he has no comprehension, which explains his lack of language. I took their report to my paediatrician (who didn't even read it being of the opinion DS2 could simply be struggling with so much input i.e. 3 languages) but did refer me onto the early intervention team. If you have one of these available to you, they are brilliant. They have interviewed me, are about to see DS2 and can access specialist SALTs and paediatricians, if necessary. For the first time, somebody actually listened.
Meantime, I have lurked on here, particularly the SN and SN education boards. From info there, I bought the MAKATON pack - have a look and if you'd like my parent pack 1, you'd be most welcome. I have almost finished it. DS1&2 love the "Something Special" programmes and I can see DS2's progress. All I need now is something similar in Spanish as everyone is recommending we use mostly Spanish with him.
HTH and you are still awake!