Jaw locks open

Every time I give my boyfriend head my jaw locks open. Sometimes I can get it to close easily other times it’s really hard to put it back into place so it will close. Tonight it was stuck for a good 15 minutes to the point where I thought I was going to have to go to the ER. It’s very painful. Has this happened to anyone else? Does any one know how to make this stop happening? It has only ever happened with him...is he just too big??? Any advice is appreciated!