9.5 Month Old Waking at Night, Sitting up Crying in Crib

Your baby is normal. They all do this.
Because, whenever a baby is going through motor movement changes or developmental leaps... it can and does tweak their sleep.
Keep in mind, that each movement a baby "learns" it does not mean that just because they are doing it that they then know the full.... complete movement of it. They do not have "coordination" yet, of their body or of all their limbs.

ie: if baby is sitting up... they do not necessarily KNOW how to then, lay themselves back down, to then go back to sleep. Or if they are pulling up or standing, that is what they do. Even at night. BUT, just because they are standing or pulling up, it does NOT mean, that they know ALL the sequencing of it. Once they are sitting up or standing up, for example, they OFTEN DO NOT KNOW YET how to then, complete it and the sequencing of it.... and then lay themselves back, down. Again. So they get stuck, and cry. They need your help.
Until they attain FULL mastery of ALL sequencing of their movements, they need your help. Even at night, to lay themselves back down.
This is COMMON, and is developmental based.
Babies, do this.
It tweaks their sleep.
They do this at all times at night.
They sit up.
They pull up or stand. But, they do not know how to put themselves back down into a perfect laying down position again, and make themselves all comfy again. They are only learning, these skills. It not being mastered, yet.
And until they do have it mastered and can do it at will... you need to help them.
Your baby is at the crawling around and sitting up stage. This is a developmental leap/change. And each change/developmental leap or milestone leap, affects their sleep. Growing pains.
Normal.
And then when he is older and starts to pull up and stand up, it will happen again. They may be standing, but they do NOT know how to then... sit down again and then lay down again, and then go to sleep again. The 'sequencing' of their movements, is not instinct yet and they do not know how. Yet. So you need to wake up and help them. Or they get stuck/stranded there, in that position and they cry.... for your help.

9 months old is a growth spurt time as well. And they also may get hungrier and more often. And need to feed.

Also keep in mind, that sleep patterns in a baby/child/toddler/adult, is not static. It always changes. Per age stage and developmental changes and per growth spurts. In a baby, the growth spurts are every 3 months. The next one for your baby will be at 12 months and so forth.

It does not seem that he is teething at this time, it is more like he is able to sit up in his crib and not able to get back down and go to sleep. So he is just sitting up crying and crying. He is able to put himself back down on his belly when he is on the living room floor...but the crib seems a bit different.

You may have answered your own question yourself. When he begins to cry go in, lay him down and leave. Do it matter of factly, no lights or excitement, just "Shhhhhh, there you go, good night." Work on showing him during the day how to lay down from a sitting position, and make sure his crib mattress is as low as it goes, he'll be pulling himself up and standing soon.

Growth spurt is the easiest answer. He's waking up hungry.

Check his mouth, teething might be the problem.

Just a phase, unless he is sick during the day. He could be cutting teeth. That would be grounds for Tylenol. It could be he is just excited about his new skills and can't sleep. That will clear it's self up, also. Just be glad he is not pulling himself up and falling backward, hitting himself on the crib rail (mine).

ETA: try a light snack before bed, if you think it might be a growth spurt.
If he has went back to sleeping at nap time, it's still a plan. Its just going to take practice. Give it two weeks, it will resolve its self. If you get involved, it will be much longer. The other thing you might try is trying a later bed time.

Is he teething? Teething was always a sleep disruptor for our kiddos.