Speak to him. Tell him that you don't want him to come in your mouth, and for him to let you know when he is about to cum, so he can take it out.

you should have discussed it with you previous boyfriend you should tell your bf first that you don't want him to cum in your mouth just finishhim with your hands saliva is an exelent lube

It is the responsibility of the man to warn you that he is about to climax and soon enough so that he does not coast and loose control. The early warning serves to following purposes:
a) so you can remove your mouth and finish him off by hand
b) so you can change tactics and perhaps use both your mouth and hand
c) so you can move around and get into position of intercourse

If he is unable to give sufficient warning and his climaxes sneak up on him then he needs to learn to have control and/or you need to use the Squeeze technique. Do a Google search to learn about the Squeeze. Do an advanced search of the forum using my name and the key word: PE or Premature Ejaculation or benchmark or exercise.

If you do not want him to ejaculate in your mouth, then he has the obligation to stop before this happens. If you do not mind having him do this yet do not want to swallow or have the semen fly to the back of your mouth, then the fix is simple. Just raise the tip of your tongue to the roof of your mouth and let the semen hit the bottom surface. Problem solved. He can cum in your mouth and you can deposit his semen discretely into a tissue, later.