Hi, I'd like to help you with your question.
Most likely, your boyfriend went back to his old relationship because somewhere in his past he was abused (most likely in childhood). An abusive type of relationship is something he feels more comfortable with because it is what he knows and understands.
This is a very common reaction for people who were abused in childhood. There is a saying- "It's the devil you know", meaning that you are more comfortable with situations you understand and know how to respond to rather than situations you do not. Being with you made him uncomfortable not because there is anything wrong with you, but because he doesn't understand how to behave in a normal relationship.
Until your boyfriend can gain insight into his behavior and understand that the relationship he is in now is unhealthy and harmful, he will most likely gravitate towards any type of abusive relationship. However, if he can see he has a problem, counseling can help him understand what is normal and how he can deal with his past. That way, he can have healthier relationships.
I hope this has helped you,Kate