You need to have a look at his uni choices and a) what they want for entry and b) what sort of career does he want?
Computers from 15 years ago are not a problem for computer science GCSE, it can be taught without a computer at all.
CS is a science and has nothing to do with changing systems, installing new sound cards etc. I say this so he isn't selecting the wrong course for him.
CS has a fair amount of coding but is a lot to do with logic gates, converting numbers to Hex, Octal and binary and back again.
About 50% is theory, you learn how a CPU works.
Does he do any programming?
If he is looking at BTEC the qualification is made up of a number of units, and these vary by provider. Usually they vary because of the teaching staff available and the skills set. Some universities want specific units to be studied eg in Health and Social care a BTEC will get you on to a nursing degree but must include 'Anatomy and physiology', if it doesn't then the uni will ask for an A level in Biology or similar.
qualifications.pearson.com/content/dam/pdf/BTEC-Nationals/Information-Technology/2016/specification-and-sample-assessments/specification-pearson-btec-level-3-national-extended-diploma-in-information-technology.pdf
The other thing with BTEC is the current government are trying to scrap them, this was discussed in parliament this week, it has been put on hold, thankfully.
The government want to introduce 'T Levels', now as far as I can tell a T Level will be useful to go from education to the workplace but will not get you into uni.
Personally I think the T Level will go the way of the GNVQ and the AVCE.
Both BTEC and A Levels have been around a long time and both are respected