Amanda needs help getting pants to fit her slender son:
My son is very tall but very thin, so I often find myself buying pants that are too big for him in the waist but fit him in the length. Unfortunately, a lot of these pants came without belt loops, so I can’t use a belt for them.
Has anyone come up with an easy way to tighten pants that don’t have belt loops without sewing!? Looking for something that doesn’t require a permanent fix.
Okay, Amanda. We’ve got some ideas for you.
Buying new? Look for pants with adjustable waist elastic.
My first suggestion is to buy pants with elastic sewn into the interior waistband that you can pull tighter and button.
Those were the only pants that would fit my slender daughter when she was younger. What I loved about them was that the elastic was invisible (it just cinched the fabric at the waist a bit) AND could be let out as she grew. No sewing required!
We had good luck finding adjustable waist pants at Old Navy. For example, here are the Built-In Flex Straight Jeans for Boys.

Old Navy has the same adjustable waist elastic for toddlers as well, and in different fabrics, like these skinny twill pants.
Tighten pants with belt loops with this no-sew elastic hack
Donna’s easy parent hack retrofits any pair of pants with belt loops, and it’s a cinch to do. Get it? A cinch? Oh, how I amuse myself.
Upon leaving on vacation and forgetting to pack a belt, I found a quick, easy, and cheap way to fix pants or shorts that are a bit big.
Cut elastic to the length you need, loop it around two side belt loops, pull tight, and tie using a double knot.
I’ve done this with my kids’ pants as well as on my own. I will just leave the elastic on until it’s no longer needed, then snip it off with a pair of scissors. A t-shirt will cover the elastic and no one will be the wiser.
Elastic comes in various widths and colors. I get the 1/4-1/2 inch width elastic. I always keep some in the car in my supply kit.

If you’re looking for something a little more decorative, Parent Hacks reader Dorothy says a length of colorful ribbon works just as well as elastic.

Tighten pants with belt loops using Dapper Snappers
A bunch of Parent Hacks readers raved about Dapper Snappers, a snappable elastic band invented to solve this very problem (by a parent, of course).

Tighten pants without belt loops using mitten clips or (if all else fails) diaper pins or safety pins

No belt loops? Try using sturdy elastic mitten clips to tighten the waistband (this works best with soft pants), diaper pins, or large safety pins.
This parent hack is featured in the book Parent Hacks: 134 Genius Shortcuts for Life With Kids.