Yes it is possible to have sperm in precum. It's a one tube fits all, so it's hard to tell what is in there and what's not. I'm not sure what extent anejaculation would have on this though- can he ever have a proper orgasm? Sperm and preejaculatory fluids are two different things. The fluids before ejaculation occurs are mostly lubricants and other stuff that is to help get the sperm along to it's destination.
After having a bad reaction to plan B myself I understand the hesitancy, but I can't give you a straight answer one way or another. I guess you need to way up the chances that there is sperm in his pre fluids and the chances that you were/are soon to be ovulating (because sperm can last several days so if you ovulate this week it could result in a pregnancy) against how bad it would be to take the plan B. Worst case senario against each other- is pregnancy much worse than being sick for a week?
In about 3 weeks time you can take a home pregnancy test with your first morning urine. Until then just remember those "early pregnancy symptoms" can be caused by stress so even if you start feeling ill, try to remain calm until you know.
Thanks for your insight! I'm still struggling with it. I think my chances are very low since my ovulation cycle is usually pretty long (24-30 days apart) and he has complete anejaculation (it's very unlikely that he has ejaculated at all within the last few days). BUT STILL, I definitely do not want to be pregnant. I think I'm leaning towards doing it just in case.
That's probably wise, maybe try to see if you can use a different e/c then last time. I know there are different kinds, so maybe another type won't be as hard on your body.