My boyfriend is undetectable with hiv. i am worried i can get it if his blood or semen gets on my open wound. since he's undetectable what's the risk?

Universal precaution: "Universal precautions" means you should always use precautions with anybody regarding body fluids and pen wound etc. Assuming your BF was HIV+ and now it is not detectable, I would still use Universal precautions, wash hands often, avoid body fluid contacts. Since, assuming he was HIV +, the information is not 100% as to whether he is able to transmit any HIV to others, so always use precaution.