In my story there is a humanoid alien race, that looks almost exactly like humans, but as you might have guessed, are boneless.
Just to be clear, I'm not interested in reasons why they would evolve to be boneless, but how they would achieve that.
What kind of muscles/membrane would they have to be made of so the moment they stand up they don't fall like a deflated balloon? ( Or stand up in the first place. )
They would need "bones by any other name", no land animal stands more than a couple of centimetres high without some form of skeleton. And banana slugs aren't exactly humanoid.
Now there is one part of my body that can get hard without a bone: the human penis. It is unusual among mammals in that it doesn't have any baculum to strengthen it, and becomes tumescent under blood pressure alone. Your humanoids have structures that contain Corpus cavernosum under permanent pressure to provide rigidity. Probably some kind of modified circulation system is needed to maintain this. The pressure doesn't have to be provided by blood (as in the penis) but could be separate high-pressure circulation system, including a second "heart".
The aliens could, perhaps even at will, reduce pressure to one of these structures, causing it to go flaccid.
Your humanoids cannot maintain their humanoid shapes without having structure-supporting structures. They might look a little bit insectoid, with jointed armor-like segments on their skin.
These bone-like spongy tissues could harden like concrete when flooded with some form of hydraulic fluid, and soften when drained of said fluid.
Make them non-deflated balloons.
You've already solved your issue. Fill them with gas instead of deflating them. The thing with gas is: It takes up a lot of space for little mass. Living beings constantly produce all kinds of gases in significant quantities because it's easy and convenient, animals most notably CO2. We even have an apparatus that constantly compresses air with our lungs + muscles. Building up some pressure is easy. So make them literal balloons.
Don't use hydrogen though, classic mistake, not a good gas. It leaks through everything and then explodes
Keep in mind: bones are not only made for standing upright, they also act as origins and insertion points for muscles and tendons.
Best approach: an exoskeleton with muscles attached at the inner side of the exoskeleton or at thorax pit like structures as seen in the opisthosoma of tarantulas.
Another approach: skin ossifications acting as endoskeleton. An example for skin ossifications can be seen in crocodiles (made of bone) or (made from calcite) in sea stars, sea urchins, and sea cucumbers.
A gas filled body is also possible but needs a huge amount of pressure...
Have you thought about a hydrostatic skeleton like the one seen in round worms like Ascaris megalocephala?
But: everything creating pressure from the inside makes your human-like creatures quite vulnerable. For example: piercing Ascaris with a needle leads to a worm-liquid fountain of 20-30 height. Things like stabbing your human-like alien would cause quite a fountain ;-) hurting the gas-balloon inside your alien would have weird effects, too. In both cases (liquid and gas filling), you need a really quick and advanced repair tool.
Lets see, what part of the body can become stiff without a bone . . . oh, I know.
Blood flow might work, a steady pressure of blood flow to maintain upright posture, which could be turned off when the body sleeps, or sits/lays down. It's not ideal. A bone maintains structure on it's own, while bloodflow requires energy, but it's possible.
Giving the planet lower gravity might help too.
