I would encourage a STD test just to be safe. You never know what someone could have from a previous relationship, or even from birth. You can even suggest that you two go together to make him feel better. My boyfriend and I have been together of 7 years and we both went to get checked at the beginning of our relationship. Better safe than sorry.
It can be his own discharge or it can precum. You can avoid this if you wash the genital organ before oral.If you do not like oral sex, tell your boyfriend frankly.I hope this suggestion helps you.