He absolutely does need work and life experience. It is blindly obvious when new cops have never had a job before and it's a difficult life or death job where you need to be a proper adult.
I'd get him to consider joining as a PC and not going direct entry. If it is absolutely what he wants to do then he does not need a masters. Perhaps continue with the undergrad so he's got a degree to fall back on if he doesn't like policing.
What I'd advise him to do now:
Join the Bluelight Police recruitment support Facebook group, it's never too early. He can get to grips with the application process and the competency values framework.
Get a job alongside his studies, in the first instance any job but with variety. Hospitality or caring would be good as flexible hours. As much customer facing stuff as possible, he will need to be able to communicate with and work with everyone in society.
Volunteering is an ok idea too, but he may as well get paid for the work that he does.
Mental health or youth work would be valuable experience. Great idea the person who said become an appropriate adult, would be a good way to see how the custody system works.
I'm not saying that you can't become a good detective without front line experience, but I really struggle to see how the new entrants will be managing complex investigations properly with just 12 weeks experience on area. I know several candidates who have transfered from direct entry dc to staying as a PC to properly learn the ropes. It's a long old career with plenty of time for role changes.
I started as someone who was ADAMANT I'd do my detective exams very quickly to be a DC. Within weeks of actually doing the job I'd changed my mind, he should not limit himself, keep an open mind, the pay is exactly the same (shockingly bad) whichever route you enter.