My father-in-law is a Chartered Engineer and very senior in his last couple of roles and I was an engineer (construction) many moons ago. Whenever we have had conversations about young people interviewing, whether for graduate entry or apprenticeships, he always, always stressed the importance of showing up looking smart! That to him, and it may be an old-fashioned concept, always included shirt, tie, and suit. If not a suit, smart dark-coloured trousers and ideally a blazer or smart wool jumper over the shirt. Think school uniform and you won't be far off the mark for an apprenticeship interview, whether for degree or 16+.
If finances can't stretch, I'd recommend vinted or alternatively, a relatively cheap suit; together with smart, clean and polished shoes. The people hiring will likely be older and even though most, if not all, engineering apprenticeships will require work clothes like overalls or very casual 'work clothes', interviews still require a more polished look.
FYI: interviewees showing up casually dressed and/or looking scruffy never impressed my father-in-law! That's not to say he won't get the apprenticeship if he adopts a more casual get-up...but if I was him, I wouldn't want to be placing more obstacles in my way, when apprenticeships are pretty competitive these days!
He may feel a little overdressed but forget about being 'quirky' for the interview, he needs to get a foot in the door! I think in recent years, especially post covid, we seem to have forgotten that for a lot of industries smart clothing is still necessary for an interview.
Hope that helps and good luck to him!