School is a very stressful environment for even well-behaved kids. Just remember all the petty insults, issues and pecking orders that we all had to cope with. Because we had no choice we had to go to school and deal with it over and over. At least as adults we are free to leave a really miserable workplace.
If there was any way, could you visit your kids at school during lunch, or maybe just the youngest? He might really benefit from being able to "download" to a trusted adult. (If not you...another relative? ) Other cultures..Italy, rural France..the kids go home for lunch with the families. It truly keeps family important and allows for some "downtime" from the stresses of peer interaction, esp unsupervised peer interaction.
You are right to suspect that your son's defensive attitude could head him into problems. If you really want to see how bad it could get, see "Hold On to Your Kids" by Gordon Neufeld. It is at the local library, or paperback.
The problem with sibs picking on him only adds to his frustration, no relief at home after returning from much of the same at school. It is most important that he knows the adults in his life are his vacation from all this torment, filled with unconditional love and acceptance of him.
I would try to re-establish some harmony at home too...trying to appeal to the older kids' (esp 12 year old) sense of responsibility ..a big part of growing up. They need to take care of their youngest family member. They are not toddlers...and can comprehend this. Unfortunately, the constant age segregation of our public school system (grades)leaves kids unfamiliar with working with or around kids of other ages. Being responsible for another child is not "natural" in such an environment, but if you look around the world, most cultures put older kids in charge of younger siblings (for limited amounts of time) with good results. Maybe some inspiring stories about families/kids surviving together...like The Box Car Children (I forgot the author)would be helpful.