I would suggest a two part solution:
Part one - a family meeting (broken into two sections, one with you, Dad and SS and one with all the kids). When it's just you, your H. and SS, talk to him about your feelings and hear his.
Ex: He can voice is big feelings. You can let him know, "I understand you are hurt/angry/frustrated/feeling displaced/etc. I feel hurt by your words and actions towards me. I love you though and we will work through this as a family."
Afterwards, when all children are present, set ground rules and expectations. Make sure they clearly stated and written down. This would include behavior and chores for all the kiddos. For example:
We don't yell/we use respectful tone of voice indoors
We use respectful language and "I feel" statements
We listen to one another
We are kind to one another's personal space and bodies (no hitting, kicking, etc.)
Etc.
Also, allow him/them to understand the discipline that they can expect when they break the rules.
10 y/o chores (ex. clearing and setting table, clearing personal toys and objects, sweeping the kitchen, feeding the dog, homework, taking out recycling, etc.)
3 y/o chores (ex. clearing personal toys, giving dog water, helping put dishes into dishwasher, mixing the salad, etc.)
2 y/o chores (ex. clearing toys, wiping down the table, brushing teeth, etc.)
1 y/o (being a baby)
Consequence for not completing tasks.
Part two:
Dad takes SS once every X amount of time on a special date (ex. once a month Dad and him go fishing, hiking, biking, something where one on one time is spent with each other.)
Once every X amount of time you and SS go on a special date (ex. fishing, hiking, to the zoo, etc. Again, this is a chance for the two of you to spend positive, alone time with each other)
This isn't a reward for his behavior, but a chance for the two of you to spend quality time and for him to feel special and recognized. It can't be easy being the eldest of four siblings when the other three are so young and are at an age where their needs are a priority.
~~When my niece came to live with us (much younger than your SS) she often stated, "I don't like Uncle ____", my husband. He began taking her on special "big kid" dates. They'd go look at boats on the dock, take a ride in the truck, practice riding bikes or hang Christmas lights. Like magic, her fits began to decrease, her relationship with him became stronger and her relationship with our daughter became less challenging. She is a kid, so her language rough and "mean", but her emotions very real. For him to give her visible special time and love helped her feel like she has a place in our family and that she is accepted and loved regardless of her internal chaos.
Now, like a said, she's a lot younger, but I think regardless of age, children are still building their verbal and emotional skills. Giving a child emotional safety allows them a better foundation for building and expanding on their "tools". It sounds like you have been doing a lot to act lovingly and create that space. I'm sure you all will get through this time. Good luck.