He has failed twice. This means he has not demonstrated that he can successfully complete the degree, so there is no point in him continuing and the university has required him to withdraw. It would be wrong for the university to continue to take money and time from someone who they do not believe can succeed. At the university where I work, about 5% of first years fail in this way and are required to withdraw.
Should he wish to appeal against this judgement, he needs to show that a situation that has been (or will be) resolved caused this failure. He must outline what the situation was and the steps that have been (or will be) taken to resolve it.
He must take this seriously and gather all the evidence he can to support his case. He (and you, if you want to help) should go through all the information about the appeals process, what is and isn't accepted as a basis for a mitigating circumstances appeal and the evidence required to support it. Usually, you are required to provide evidence about why you failed and evidence about why you did not present this evidence before you failed.
The last chance June resit will only be granted if the panel have strong evidence to believe that there was a resolvable reason for the failure and that there is a plan in place to ensure that the outcome will be different in June and going forward.
It is possible that they might consider that the lack of support provided due to Covid is the sole reason for the failure, but it is also possible that they wont. Based on what you have said, as someone who sits on appeals panels, there is a considerable amount of hard evidence that suggests that he was unable or unwilling to engage with the support processes that were in place to help him, including his lack of prior engagement with available support, lack of external evidence of his declining condition formally presented through the correct processes at the time and not having applied for mitigating circumstances on lack of support or any other grounds before he did his assessments, all of which have contributed to his failing. Consequently, it would not be unreasonable for the panel to reject the appeal on the basis that it lacks sufficient evidence in all relevant areas. Most appeals are not successful.
The appeals panels are process driven, they deal in hard evidence, nothing else. It does not matter how much we understand that it is difficult to have ADHD or to be young or to fail by two marks or anything else. There is only one question: Is he equipped (in terms of ability, support, etc.), right now, to study successfully for this degree? If the answer is 'no', we might be very sorry that it is 'no', but it is still 'no'.
He (and you) must not assume again that everything will be ok because his tutor thinks it probably might be. He must take responsibility for doing what he can to make it ok if that is what he wants. You both seem to speak as though he will just be allowed to resit in June and then he will return to his course, but this is far from certain, especially if he just drifts along expecting it to be so.
In terms of next steps, he must think quickly about what he wants to do and, if that is to continue with university, he must think what will enable him to do that and what support he needs. I would suggest that if he wants to still try university, he arranges to speak, as a matter of urgency, to the Senior Tutor, Director of Undergraduate Studies or equivalent in his department. If you want to support him, help him to come up with a list of questions to ask them.
These might include: Is it possible for him to resit all or part of the first year (with or without credit) to help him to establish his support systems and good working practices in a lower pressure situation? What support can the department provide in his appeal? What support can the department provide if he is successful in a June resit and can he work with them to produce a plan (ideally presenting some ideas before the appeal)? Alternatively, if he has already come up with a plan for the kind of support that would help him, would the department be able to do the things he thinks would help? If he decides not to return, can he get any kind of credit, certificate or qualification based on what he has done so far or with some additional work?
Given that he has a history of forgetting things, I would suggest that you make yourself aware of the relevant deadlines for the appeals process, re-enrolling, etc. At the university where I work, you must submit your appeal within 10 days of receiving your results.