On the face of it, his complaint relates to 'failure to deliver a service'. However, you/he need to work out where the root of this failure lies.
There seem to be two issues. One is his inability to access some kind of teaching related to his dissertation. Here the question is whether the root cause of this is something your son should have taken responsibility for or whether the department should have. From what you have said, it appears that it is in fact something your son should have been responsible for, as he is responsible for ensuring he has appropriate equipment to access the course. The department could have helped, but it does not constitute a failure to deliver a service that they did not.
The second issue is his lack of dissertation supervision. Here we need to understand how that arose. If there was an expectation that your son would access the relevant information (such as the information that he should have a supervisor which it is difficult to believe he was unable to find anywhere in any course material), follow a process for being allocated a supervisor and make contact with said supervisor, and your son did not do these things, then this is not failure to deliver a service because the service was available but your son did not avail himself of it.
So here we need to understand whether your son followed the correct processes for being assigned a supervisor and trying to arrange supervision and whether he submitted his draft to be read at the appropriate time (and whether supervisors will read a full dissertation draft - we don't, we will only read and comment on sections of the dissertation and only at a specific point early in the process due to this being an independent piece of work, so sending a full draft at an inappropriate time is a pointless exercise and whether or not his supervisor was on holiday when he sent it is irrelevant). If he did not access information that was available to him and follow the correct processes, then again the department could have been more helpful, but it does not constitute a failure on their part that they were not.
So you need to consider these things:
- What is the specific failure your son wants to highlight? What has the department done that contravenes university regulations? Keep in mind that the complaints procedure is a technical exercise, it relates to compliance with policies, regulations and rules on the part of both the department and the student.
- What outcome does he want to achieve? Does the university have a policy relating to delivery of service during the pandemic and whether or not this can be used to apply for an adjustment of his marks? If it doesn't, then what is the outcome he wants?