Okay, I did my best to choose some T-shirts and shorts for each boy in the shops you said. I also added a jacket and a pair of trousers with each boy (except Niall), for when it’s a bit colder this summer ;) I’ll give some tips too with each boy!
For Harry I’d choose colored shorts (happy colours like pink or orange…) and normal shirt or shirts with lace on it. Nothing too much, just simple but very clean :) As jacket I’d say a blazer or a parka and trousers are very skinny and more the dark colours (like dark bue or black).
For Louis it’s pretty easy: shirts with stripes! Colours who are important for Louis are navy blue and red. So, red and navy shorts. Also longer shorts are cool for Louis. Also jeans shirts are good. As jacket I’d say denim. That’s Always good!
For Liam it’s more a high school style. So, shirts with prints (for example numbers) are Always good. Also shirts with sleeves in a different colour (like he wears on the picture) are very ‘Liam’. Essential are checkered shirts, preferable with a simple white T-shirt underneath. Simple jeans shorts are perfect for Liam, but briht colored ones (I mean pink or beige) are okay. Also trousers in bright colours. As jacket I’d say some normal jackets, nothing special (the jacket on the picture is an oversized parka)…
For Zayn I’d go for dark colours and white. So, dark shirts or white shirts with a dark print. Of course colored shirts are okay, but keep it a bit darker… For shorts I’d say jeans shorts, but damaged shorts (you know, with cracks and bleached) or black shirts. Or black and white striped shorts. For trousers, you can Always wear jeggings, or black skinny jeanses of course. As jacket you can go for a leather jacket, a Varsity jacket or a sleeveless denim jacket.
For Niall it’s rather easy: sportive. A regular jeans short with a simple T-shirt are the best. Of course you can go for a more special short or a colored one. Just a regular T-shirt for Niall… Or a jeans shirt with a shirt under it, but that’s already less Niall. As a jacket, I’d go for a baseball jacket. Light colours for Niall, nothing too dark…
As you see, jeans shorts are good for every boy! Just like skinny jeanses. Also chinos are good (especially for Louis and Niall). Jeggins are good for Liam, Zayn and maybe also for Niall. Parkas as jacket also good for every boy. And with a regular, basic T-shirt, you’re Always good!
I hope I could help you a bit! The cloths on the pictures are only from the stores you told me (Bershka, PullandBears and Stradivarius). I hope my tips can help you to choose for yourself in other shops ;) If you have questions, just ask me!! (Oh, I added the link to the pictures, because they aren’t really big here, so…)