My bf pulled out to cum and as he was finishing himself off I noticed a liquid coming out at the base of his penis? Was that his cum?

Discharge. Assuming that your partner has normal genital anatomy, there should not be any emissions from the base of the penis. However, if there are any skin lessons or pustules, those could erupt and produce a discharge. Lastly, it may be that some of the ejaculate dripped down into that area.