N., I couldn't help but relate to this situation. My son went through something very similar around 18 months old. I kept wondering what had happened to my little angel baby that he would suddenly become this little monster. I tried to keep my patience, because he had been sick - like your son. But, finally I called the doctor which led to a discovery. My son has diabetes! I didn't think much of the excessive thirst and leaking wet diapers because he was recovering from strep throat and I figured his throat was just sore. But it turns out, his poor little body was literally dying of dehydration in an effort to rid his body of all the excess sugar in his system. A very simple test confirmed the doctor's suspicions and he was quickly admitted to the hospital where he received immediate care. They saved his life and he returned to the happy little kid I had known before. I felt terrible for thinking he was a monster when he was just so sick and couldn't talk to tell me how he was feeling.
So, my point is - call your doctor. It's so much better safe than sorry. Maybe there is something physically wrong with him and he simply can't express how he feels with words. Have your doctor check him out head to toe and tell your doc about any strange behaviors or symptoms. I think it's rare for little ones like yours to act out for no reason. It's usually something physical.
Good luck with your precious baby. I hope you can figure this out and that all turns out well.