Can you get pregnant if your boyfriend only puts it in your bum?

Highly unlikely. All though, if the sperm gets near the vaginal opening, there is a slim chance it could get inside.

I agree with the slight chance if sperm gets into your vaginal canal from ejaculation or rubbing of sperm near your vaginal opening. Highly rare, but it is not a 100% impossibility.

Again, it shouldn't matter where your partner puts his penis--whether it's in your vagina or your rectum, he should be wearing a condom every time.