Sorry A., you won't get any encouragement on letting your little one cry it out from me. In my opionion babies (and at 9 months he is still a baby) cry because they NEED something. He is hot, cold, just needs to check in with mamma, hungry, soiled, wet ... something.
It could also be he needs fewer naps during the day. Babies around that age sleep around 12-14 hours in a 24 hour period. So, if he is taking several naps, then he is using up some of those 12-14 hours.
Also, if he is working on a new skill...walking, standing, pulling up, whatever that often disrupts the nighttime schedule. They are so anxious to try it that when they wake up it throws them off. Growth spurts OFTEN disrupt the night schedule. And if it is a growth spurt (which is TYPICAL around 9 months) then he probably needs MORE FOOD to fuel the growth. My daughter went through a growth spurt around 9 or 10 months. She grew an inch and a half in 2 months!!!! It seemed like she ate around the clock. I was EXAUHSTED ... but then it was over and now we are back on schedule.
Babies are NOT little adults. And we are FOOLISH to try and squeeze them into a box of being an adult. They will not sleep like an adult, they will not eat like an adult, they will not react like an adult.
Honestly, I would try to work with him a little more and try to figure out what the actual problem is. If you just let him cry it out you are not addressing the real issue and you are just teaching him that his needs will not be met. Babies are not manipulative. Figure out what he needs and I'm sure he will be much happier.