Teen boy - semi-formal

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We have an event coming up May that we will need to be fairly smart (but not suit smart) for. DS will be 12 but is tall and slim so tends to need trousers a couple of years older than he is to get the leg length. Lots of kids ranges seem to stop at age 12 but small men's swamp him.
I'm looking for a smart-ish shirt and trousers for him but I'm struggling for inspiration. I don't want to spend more than £75 all in as he's unlikely to wear it more than a couple of times.
He isn't particularly clothes conscious (not into brands, shows no desire to chose/buy his own clothes and is happy to wear whatever I suggest for him so not struggle to get him into something smart).
I thought I would be able to grab something from Next but they don't seem to have longer leg lengths and none of their shirts have grabbed me (they are all either plain or a bit 70's garish). So any thoughts on where I should be looking?

If DS14 needs to be smarter we go to Hollister
He has some Chino type trousers from there and nice shirts, he’s around 6ft

Have a look at Jack Wills. Everything there is so long.

Kin do some quite smart shirts. They are mens’ not teenagers’, but a small could work.

I know a lot of people tend to let their children wear their school trousers (generally plain black) and then just buy something for them to wear on the top half.

When my kids were younger, I always found the teenage years difficult as children's clothes were too small and adults were too big!

H&M or Primark have smaller mens sizes and teen stuff that might work.

It all appears to be joggers/hoodies I'm looking for something smarter.

We have had smarter clothes from Jack Wills too

Uniqlo usually do narrow mens chinos with small waists and long leg lengths

www.uniqlo.com/uk/en/product/slim-fit-chino-trousers-455482.html?dwvar_455482_color=COL42&cgid=IDchinos2382

Lots of shirts to choose from too

Even the smallest men's sizes swamp him - he's 5'8 but still fits into aged 8-9 stuff in terms of width (nothing ever actually fits - it's a permanent compromise between length and width).

The uniqlo ones start at 28 waist.

Is that too big?

Next do chinos up to Age 17, in skinny and standard fit.

They also a lot of semi-formal shirts in these sizes.

You need to go online to find the teen sizes.

Tommy Hilfiger and H&M men's stuff tends to be long and lean too.

M&S do smart trousers up to age 17-18. I used to buy my sons school trousers there. They still have the elastic and buttons inside the waist band for the more slender teens!

Zara have nice teen clothes and the men’s xs can be the right size depending on the boy. Lots of semi smart chinos, jeans, trousers and shirts

I found River Island good for small waists and long legs.

Have you looked at next men's sizes? DS is tall & very skinny, they are one of few places to do small waist sizes, can do short leg length if needed. Def do 28" I think possibly 26 as well

Some Uniqlo mens trousers are sold in sizes XS, S, M etc

Size guide says XS is 26-28. Is that any good?

I've had a good look at Next there a few things that will do at a push but nothing is grabbing me. I was hoping for a subtly patterned shirt - striped or paisley or something but they are all either plain (so look a bit school uniformy) or garish patterns or really casual (checked flannel).
I've always struggled to find things that fit (where do you get boys shoes that aren't trainers or school shoes once they hit size 6+ until they hit 8 where the men's sizes seem to kick in?)
Casual clothes are fine if they are baggy but if he is totally swamped in order to get the length he won't look suitably smart.

A few possibilities there thanks.

www.jackwills.com/ainsworthy-stripe-oxford-shirt-553517#colcode=55351715

www.jackwills.com/gentlemen/clothing/trousers-and-chinos

Mine acquired (dirt cheap) a beautiful Hardy Amies dinnerjacket (charity shop) which he wore to all occasions with a shirt, black bow tie, red braces, and levis. Girls swooned.

Next and M&S are the only places I can get trousers and jeans for DS. He wears the slim fit stuff.

XS mens from River Island should work

Next kids trousers in a size or two up but in a slim or skinny fit so they fit at waist?

Anything suitable in the Mango teen range?

And a pair of vans or converse in a matching colour is totally fine for that age.

That is exactly the sort of thing mine would love but the occasion we have coming up is no jeans (it's a military thing so we have to be smart enough to be allowed into the mess) but it's not a suit/tux occasion - that would be easier as loads of places to suits in teen sizes but there seems to be a real gap for early teen. I know it's the current fashion but teen ranges seem to be all joggers and hoodies. Men's trousers in 28" waist might do (or M&S xx long boys if he is OK looking like he's in his school trousers) but men's shirts even in the small size are far too big on him.