PrEP and PEP are not quite the same. PrEP is taken on a regular basis before exposure to HIV, PEP is taken as an emergency measure after being exposed. PrEP only "protects" to a certain point...if you fuck him bb, there is a good chance that your HIV will get in to his blood stream and infect him. With PrEP, the hope is that the meds will limit the ability of the HIV to take hold and multiply. As I understand it, It could still be possible for it to get in to some places and hide, so if he goes off PrEP, there is a chance that what little bit of HIV may have survived could come back and get him. At least, that is my unprofessional understanding of it.