Teen boy - semi-formal

[OttersMayHaveShiftedInTransit]

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?

There are a couple of possibilities there - typically we are bloody miles from any of their stores as it would be really handy to be able to try stuff on. With three months to go he could easily grow another 2" between now and then.

Would it be worth hiring something?

I’ve no idea of the costs nowadays but if he’s only going to wear it once before he fills out and can no longer fit the small waist then it might be more cost effective?

Does anywhere hire stuff that's not suits?

If you've got three months to go I would hang fire - there'll be more formal-ish clothes in stores as wedding season hits so possibly a greater range of trousers/shirts that might fit the bill. Takes away the risk of growing out of things too.

I unexpectedly got my tall skinny teen chinos and a linen mix jacket from H&M which were a great fit, having gone through virtually everywhere else known to man.

He sounds very cool!

I'd have swooned 35 years ago!