Hi, I'd like to help you with your question.
Whether or not your boyfriend is jealous, his cruelty towards your son is harmful. He is putting his needs before your son's needs. And he is picking on a child, someone who cannot defend himself.
It is important that your boyfriend understand that his behavior is harmful. You mentioned that the counselor who saw you both tried to get this point across to him. But apparently, your boyfriend either does not feel it is important enough to stop him from acting this way, or he has issues that he chooses over your son's needs.
Keep telling your boyfriend the harm he is doing. Children who are verbally abused often grow up angry and insecure. They struggle to feel good about themselves and often end up hurting themselves or others through their behavior. They can also become "repeat offenders" through hurting their own children. Your boyfriend's insecurities can cause a chain reaction of pain through your family for generations.
Also, point out that your boyfriend's need to abuse your son says that he has issues he needs to address. An adult who picks on a child needs to get help to find out why they desire to lash out this way and hurt someone who cannot defend themselves. Insist your boyfriend get therapy and support to stop his behavior.
When your boyfriend abuses your son, it is important that you intervene. Assure your son that your boyfriend's abuse is not about him but about your boyfriend's inability to handle his own problems. Tell your boyfriend to stop his abuse immediately and leave. It may make him angry or upset, but boundaries need set now so your son is not harmed further by your boyfriend's actions and he can see that he is not powerless.
It is also important that you get your son therapy to help him cope with how he feels and the damage done already by this abuse. If he does not get help now, the chances of emotional damage are high and the consequences harder to fix later on when your son is older. Talk to your son's school counselor about a referral or your son's doctor can help. You can also search on line at http://therapists.psychologytoday.com/rms/.
I hope this has helped you,Kate