have you spoken directly to CEAS? (ie not just with the school)
what additional help do you think he needs?
is he on school action/ school action plus and failing to meet his IEP targets?
does he even have an iep?
what are your areas of concern/ specific things he needs help with? (i'm guessing social skills, but could be anything lol... sensory? does he see OT or any other therapy provision?)
in theory SCE has the same staus as an LEA wrt statutory assessment, so if you would be requesting stat assessment in the uk (if, for example he has been consistently failing to meet targets on a SA IEP) then you should be able to request assessment in situ. i'm unsure why they would delay - unless they believe he is getting the support he needs?
but going back a step - have you had regular meetings with his teacher and senco to discuss the additional support he needs? has he been dx during this posting? is it a 'new' or longstanding dx? if he was dx prior to this posting were his needs registered on dh's personal file? (not sure what service lol...) if so then ceas should have negotiated support prior to approving the posting... have the school promised support and then it has not materialised? or do they not agree that additional support is needed?
(not in cyp lol, but have just negotiated an o'seas posting with a statemented child...)
that'll be lots of q's - sorry!