If your boyfriend has a detectable level of HIV in his system and if you're not on PrEP, then you would be at some risk with his cum in your bum. If your partner had a detectable viral load, but you were taking PrEP, then your chances of getting HIV are nearly zero.
PrEP is a daily pill that prevents HIV by more than 99% (). You also don't mention if you are taking PrEP to prevent HIV.
Even if they are undetectable for less than six months, their chances of passing on HIV to their partners are significantly decreased. If someone is HIV+, but undetectable for six months or longer, they cannot transmit their virus to others ().
