I was born in 1970 and grew up in Manchester, so I guess that’s me then? One caveat - the ‘alternative’ music scene was thriving in Manchester in my teens, so my perspective is firmly there.
Clubs: quintessential was the Ritz ballroom, but only on a Monday night reallly. Otherwise there was 42nd Street, Jilly’s/Rockworld and the Boardwalk. To see bands he would have gone to the Boardwalk if they were new bands, or the Apollo if they were more established. The Apollo was in Ardwick Green, very slightly out of the city centre and rough as you like. Before the clubs your man should have gone to the Salisbury, fabulous pub under the arches of Oxford Road station. (Genuine local colour you can borrow if you like: there was a man with a rolled up rug under his arm who drank every Saturday in the Salisbury.)
Shopping: the Arndale for mainstream stuff, but the best options were Affleck’s Palace, round the back of Piccadilly.
City centre streets: Oxford Road, Piccadilly (railway Station and Gardens), King Street, Cross Street.
Manchester is NOT Liverpool and the rivalry is often fierce. Scousers are chirpy and say ‘la’ and ‘ed’, Mancs are dour and sharper, more cynical. I’m racking my brain for some slang and struggling, I must say. ‘Our kid’ has been mentioned, for proper Mancs your mother is your ‘mam’, if you are going into town you might ask if I want to ‘come with’. We use ‘dead’ (meaning very) all the time. The Happy Mondays are a good source, as mentioned, or New Order, or even the Royle Family, if you can get a hold of it. The Royles are slightly more Salford, though, a distinction which might be important to your boy - Salford and Manchester are adjoining cities. Salford people will tell you that Manchester stole their land :).
Others have mentioned some of the bands, I’d also add James, the Inspiral Carpets, the Charlatans. The Madchester thing was huge in 89 and onwards, if that’s your time period your boy would not be able to escape it. Always summarised to me by my favourite t-shirt range (bought from Affleck’s Palace, natch) ‘And on the 6th day, God created Manchester’, swiftly followed by ‘On the 7th day the clubs closed, but God stayed cool’.
Bad bits of the city: Hume, Longsight, Moss Side (got worse later), Ancoats. Bad things in the city: weed (although almost always resin - I never saw grass until I left Manchester) and speed were the illegals of choice, at least until Madchester in the very late 80s.
I could do this for ever, so feel free to ask if there’s anything specific you need. You’d be absolutely safe on your own by the way. It’s a great place, but then home should be, shouldn’t it?