How could I get my boyfriend to trust contraceptive gel?

So me and my boyfriend have been dating for a year on the 16th of October and we had sex three months into our relationship.we used condoms and stuff but recently we bought contraceptive gel and we used it once and he came inside me and it didn't fail. But ever since then I can't get him to use it and actually cum in me. We'll have sex but he'll just cum on me and I really want him to cum inside me. How could I get him to trust it?