i have no idea what to do

my boyfriend went out with his friends tonight and while they were driving he butt dialed me and i overheard him talking about his ex and he said some pretty gross things.. he kept calling her a cunt and saying she was fat and then stuff about jer mom (i didnt hear everything perfectly) but this is really out of character for him. ive never heard him talk that way even with his friends and it just bugged me a lot. do you think its just him being a guy or is he lying to me about the type of person he really is? i confronted him about it and he got mad at me for “eavesdropping” and said that was the first time hes talked like that and he just wanted to let loose and have fun with his friends. i really love him but it bothers me so much that he would talk like that about another woman.