The National Student Survey is the source, @Tree543, I have mentioned in earlier posts in this thread and I provided a link in the first of those. This data is also the basis of much of the informaton on the DiscoverUni website. Be careful about drilling down to the level of individual courses as the sample sizes are very small (though that doesn't stop university marketing departments using the data in an outrageously selective way!). However, at the institutional level (eg all Exeter undergrads) and the department level (eg all Maths undergrads at Exeter), the sample sizes are much more robust. Some academics are dismissive of the NSS but then it can reveal some unpalatable truths.
There is no single 'best' source of data, but @NotBabiesForLong offers very sensible suggestions. I would simply add that, at Open Days (in-person or online), encourage your DS to ask specific questions about the recent past (eg what proportion lectures and what proportion of seminars/work groups were in-person last term?) rather than asking about future intentions, which allows for vague, caveated answers.
Many universities have a 'chat with a current student' facility on their website and, again employing the 'specific questions about the recent past' approach, you are likely to get candid and meaningful answers.
There may be alumni of your DS's college/sixth form at the universities he has in mind and he may be able to ask them about their current experiences. It all builds a picture.
Individual student experiences vary not only by institutions but also by subject area and mix of modules taken. So your DS should take care not to latch on to just one or two pieces of experience, data, opinion but rather look at lots of different aspects in the round. Many MN posters say that in-person Open Day experiences helped shape their DC's defining 'gut instinct' about preferred choices.
Finally, university is largely what you choose to make of it. It you go with the mindset that you will make the most of the opportunities and will adapt and cope as best you can with aspects that were not as you expected, then you are well placed to succeed at university and beyond. I hope that helps and good luck.