Change in the home environment can be very difficult for small children. It is likely that your son is targeting his frustrations at your boyfriend because he associates him with the disruption in his home life. It has nothing to do with what your boyfriend has or hasn’t done.
Have you tried integrating your boyfriend in activities outside the home environment, such as a trip to the zoo or a picnic in the park? Removed from the stress of the house, your son might more easily come to see your boyfriend as someone who is fun and supportive.
It sounds like you have a good relationship with your ex-husband. Would he be willing to help out? Perhaps you could invite him and his fiancé over for dinner so your son can see that the new family structure is still loving and supportive. If this is not possible in your home, maybe you could get a babysitter for your boyfriend’s children and the three of you could go to your ex-husband’s house for dinner. It is important that you show your son that the new family dynamic is supportive and caring like the old one was.
Family counseling is another good option. Your son needs a safe place where he can share his feelings and receive guidance from a therapist about the changes he is experiencing. It will take patience and time, but he seems receptive. After all, he says that he loves your boyfriend “a little.” This is a strong step forward. Do everything you can to reinforce these statements.
