SLCN: A difficulty in understanding?

The following case captures a number of common occurrences that I come across in schools. I want to hone in on one particular aspect because it is something that is often missed or, if it is spotted, it is considered a secondary factor when in actual fact it should be the starting point:

A student arrives into your pastoral responsibility in the middle of the term as part of a managed move arrangement that your school has with a neighbouring school. This is meant to be a fresh start and you do your best to meet the parent/guardians, lay out your expectations and you put in some transitional support, which includes an older student buddy and a teacher-mentor. You hold your breath.
It’s 10 days when the first infraction happens. It’s in maths and you think that the teacher probably had something to do with not handling the situation well, but nonetheless the student did seem to be attention-seeking and/or work-avoiding in their behaviour.
So you chat with the student and after a while it occurs to you that his speech is unusually slow, that he’s just avoiding you in conversation, or that he seems to have a bit of a speech impediment (but only with certain words).
You try to find out slightly more about the maths incident, but he doesn’t want to talk about it and you can see that he is good at avoiding the subject (either by shrugging, silence or by simply saying “dunno”).
The same situation occurs two days later and this time in English. In the subsequent conversation you notice that he’s using the same simple words to describe the same scenario. You ask him about English in his other school and he can’t remember what they studied.
You become more aware of the same really laboured conversation and perhaps a lack of trust – which is not what you normally get from your other students.
You set him up with some private study in your office and he doesn’t do it, he messes around and you think to yourself that he may well have some form of SEN. You email the SENCO, but while SEN assessments are in process, his behaviours repeat and spiral out of control quite rapidly. Soon, your hopes for that fresh start have vanished.