Both of you can watch the movie "Hard Anal Fuck and Snowballing". You can go to PORN.COM and show him some snowballing porn films. If after talking he still refuses, then you may want to try another tactic.
Send man juice and saliva back and forth and see how many times you can do it. If in doing it the first time, everything worked for you, then you can gradually begin the snowballing process. Try to start by just missing him after cumming, so you do not have to go from zero knowledge to being all-out about it. Then, if you both agree that snowballing is something you want to try, begin by giving him a blowjob and let him cum in your mouth. If he is not familiar with the term, he might as well be patient enough to explain it to him. Make sure that both of you are comfortable with snowballing. If this is something you would like to try with your boyfriend, the first thing to do is to talk to him about it. The research was made and they found out that cum carries symbolic meaning for gays it represents the successful completion of sex. It was first used exclusively by gay men. The term snowballing originated in the United States. There is one difference between snowballing and cum swapping though, and that is with cum swapping, the cum may be passed to another dude, and not just between the dude and the original owner of the cum.
Snowballing is often interchanged with "cum swapping". You can watch some snowballing porn to find that it is very simple. It may sound complicated, but it actually is not. Then they continue to kiss and pass the semen back and forth together with saliva, having a snowballing effect as the fluid gets larger and larger in volume. Snowballing porn is a genre wherein you will see scenes of a dude ejaculating into the mouth of another dude's mouth, then kisses that dude with an open mouth, and has his partner pass on the cum plus the saliva back into the dude who owns the cum.