I had a quarrel with my boyfriend, and he said he would be a dog if he came to me again.

It's just angry words to say that you will be a dog if you come again. It can be seen from his angry words that he doesn't really want to break up, at least not very firmly. Otherwise, you won't call a dog, but you will be more decisive. He came to see you again, which means he is still interested in you, or he still wants to get back together with you. What you say when you are angry is not true, especially when couples break up. So don't be too serious. If you are still interested in him, give him a step down and take this opportunity to get back together with him. Looking over old scores or something can wait until we make up. Of course, if you are not interested in him, then you don't have to pay attention to him. He doesn't keep his word, so consider him an ex.