My DS has used a kozy carrier since he was 3 years old. The is a very skinny kid and the small belt has always fit him. It is heavy duty enough for all his playing outside.
We started with trainers also in the beginning.
Edit to add - the duel one fits 2 epis with lots of extra space, but I have no idea how large a spacer and an inhaler are so I can't comment on if it will fit that

We do have an REI very close and an even bigger one on the other side of town near my SIL.

I never thought to look there.

The pencil pouch wouldn't work for DS1 as he is height challenged as well - I think it would hang down too far to be comfortable or be knocked around consistently.  But it's definitely an idea for when he gets older.

And a drycleaner with an alteration service might also shorten straps for you, on the spot, while you wait, with son there for fit,

and for not much money.

I've also heard that shoe repair shops can shorten the straps to fit if you can find a pouch you like.

A KoziEpi Quatro would work and it could be worn as a cross-body pouch, waist pouch, or caribeaner bag.  www.koziepi.com

Can you get to an REI easily?  There may be something there that would work, though you'd have to shorten and then re-sew the straps.

DS keeps a Staples neoprene pencil pouch attached to his belt loop (it has a carabiner on it).  I'll try to find it and post a link. The only problem is that he needs to always wear something with belt loops.

So we are working on trying to have DS1 carry after the winter break.  But I am struggling with the search function.

Ideally - I need a pack that is rugged, large enough for two epis (he will carry trainers initially), a spacer and an inhaler.  But the hardest part, he is ultra tiny.  Wears size 5 pants, slim, with adjustable waist pulled as tight as can be.  I want to teach him to wear it backwards so it doesn't come off when doing bathroom business.

Any thoughts?