Agree it's really tough to advise. Really tough. My daughter got an unconditional, and although her grades were fine, she opted to reject it and go for a higher ranked uni, which caused a huge amount of stress in the lead up.
I think he needs to realistically look at what he can achieve. Was he coasting, and he just needs to put the work in, or is he genuinely struggling with the subjects. And then look at the focus of each uni and their degrees, their teaching styles. Also would an unconditional mean he coasts even more if that was the issue? Like he has a place so why put in the work?
Then try to work out what is realistically best. Is a math heavily focused course better for him, or something with more self study, does he need the support, and less math focus or whatever, It's a judgement call, but it's such an important one, and a really tricky one to make.
He's got some time, so he can spend some time making sure he understands teaching style, focus, etc of the different units for computer science, and also understand why he is currently failing, and really does he have both the appetite and ability to pull it back.