So much depends on which university your DS is attending and what their regulations are generally. Also what their regulations are for this year in particular. I work as a teaching co-ordinator at a university and this summer there have been a lot of changes specifically because of COVID-19. He should have access to a lot of information about his academic position - probably in his university Inbox. If he needs to engage in any additional work to complete the year, he should have had that clearly laid out to him. At my institution we're starting exams again next week (although in a normal year, it would be the last week in August).
Repeating the year is not necessarily an option. At my institution you would have to have very strong grounds for this e.g. serious ill health and would need to put in a formal application with written evidence. A repeat year can give an unfair academic advantage unless the student had had such a bad year (poor attendance, not submitting work) that NOT allowing them another chance at the year would be unfair.
Resits for any failed modules should always be an option, and indeed a requirement. It's crucial that your DS finds out what these involve, when they take place, etc if he hasn't already. Depending on the circumstances these may be capped (can't achieve anything above the basic pass mark) or uncapped. If he's had information about resits and doesn't engage with them, he could risk being kicked out.
Be cautious with the 'no detriment' thing. It is not a universal policy across all institutions this year. My institution does NOT have this policy - instead we cancelled all May/June exams for all but a handful of students. Anyone unhappy with this could opt into open-book online 'exams' in July/August. But having opted in, their results will count regardless of whether they are higher or lower than their existing grades.
If your DS is unclear about anything, he should make immediate contact with his departmental teaching co-ordinator / academic tutor etc for advice. If he sticks his head in the sand and misses any compulsory assessment this summer, he will be in a VERY precarious position. Not having read your emails properly, or having ignored them, is not an excuse. (Students who do this are the absolute bane of my life - I spend huge amounts of time and energy going above and beyond to support my students, and am very happy to do so, but the ones who just stick their head in the sand and ignore their problems drive me insane. The longer they leave it to say/do something, the less I'm able to do to help them.)
Re the January exam, I would expect him to have been able to submit a case for exceptional/mitigating/extenuating circumstances. If accepted (medical evidence would be needed), he would have probably been granted another 'first sit' (not resit) of the exam. Or even possibly had it excused, meaning the grade would not count towards his overall score. But this process will have been done and dusted at least a few weeks ago, so if he didn't engage with it then it will be too late now.
In my experience - having done this job for 14 years and counting - it is very rare that a student simply isn't academically talented enough to achieve at least 2:2 level (C, or 50%) grades. If they are struggling to pass, it is something other than academic ability causing that. Often, but not always, mental health issues. Could that be a possibility for your son? Anxiety and depression rob students of the exact strengths they need to succeed at university: self-discipline, self-confidence, ability to concentrate. Even one bad week then leaves them feeling unable to catch up, and everything spirals. The sooner a problem is disclosed, treated and supported, the better the outcome.