Minimoch,
I am so sorry for your loss and that your son is struggling.
We have a dd2 who has Down syndromme, sometimes we can't tell if her behaviour is to do with the Down syndromme or just being a teenager. I wonder if possibly (and I have not experienced what you have been through) there could be a complicated mix of grief and being a new student, perhaps his grief amplifies the feelings that he would have had. Unfortunately I suspect that like us it is impossible to tell and you just have to find your way on a daily basis.
I have just listened to the BBC Woman's hour clip on I Player, that Marianne posted the link to. There was a segment on Monday and one on Tuesday. In Tuesday's clip the counsellor says that the biggest problem she sees is anxiety, in Monday's clip they talk about how new students find it difficult to balance social/academic and looking after themselves.
As others have said I think it would help if you could visit him, I am sure that he would find that reassuring. It may be worth exploring why he feels the need to change course a little more, as someone else has mentioned it could be that he feels unsure of himself and could be more to do with university having a different style of academic work than the course itself.
Both my dd1 and nephew have just started at different unis, in self catered halls. We expected our dd to feel anxious, but it has been a surprise that our extremely bright and confident nephew (AAAB, who is also studying engineering) has also said how tired he is and what hard work it all is, now that he is 3 weeks in. So maybe a visit and a little practical help might be good as well. I have already got a date to pop up to see my dd to take her out for lunch and I am also going to take up a couple of meals for dds freezer etc, I know that she will appreciate them and I think that the first term is very hard and having something that just goes in the oven and a few treats may help.