I'm not sure that evidence will be any useful given the massive variance in courses and standard of work expected. Take even the mode of study - for the same degree (Computing). Some unis are very theoretical and exam based, others are more project based.
My autistic DH who was terrible at exams (CDD at A-Levels) came out with a First at his mainly project based uni. 'Difficulty' of coursework aside he deals with highly complex systems daily at work. He's certainly very capable.
For my own degree (accounting) the difficulty of exams vary highly. Some universities have a large proportion of grades coming from pop quizzes (which are very similar to what's asked in the textbook). Or 'reports' which are something like. List 3 instances of accounting events in the media and the consequences. Instead of 'find financial statements/accounting regulations and do a detailed analysis'. The depth of analysis required in the former is much lower.
On a more cynical note there are many, many qualifications that are equivalent to a degree or even Master's (NVQ Level 6-7). A 'degree' from university used to be for the academic. Highlighting an 'academic' skillset.
But a few decades ago we decided we wanted more 'graduates', converted a lot of polys (? not sure what the term is). Suddenly everything was made into a degree, and everything else was looked down on.
This means that a degree alone is no longer an indicator of anything. Beyond a few narrow fields. Employers have their own online tests, recruitment processes etc to determine the capability of candidates.
AND, more importantly a degree has become secondary to extra-curriculars that show employability. Maturity, teamwork, resourcefulness. Interviews are competency based and if you do not have good examples you will fail the interviews, regardless of how stellar your grades are.
By the way university exams are moderated highly and a 2:1 is an 'average' grade. Failing students and 2:2's look bad, I think most people, having chosen the right university for them will be able to get at least a 2:1 unless they have done zero work.
It's good that your son has outside interests. I have interviewed many 'smart' people with no common sense. There is an obsession with 'academic intelligence' but employers are starting to realise it's not the end goal. Especially as in the future... a lot of things will be automated. but the need to deal with people. To engage , inspire and get them doing what you want ;) that will never go away. And it cannot be 'taught' easily...