Hi lovey, such an awful situation for you which should be such a happy time! I no it is possible to become pregnant from foreplay if some sperm was on his fingers before hand, but from what I've read about sperm, it does not last long once exposed to air, and I would think the chances of you being pregnant from your co-worker is slim. It might be an idea to tell your co-worker your predicament, and he may be willing to do a DNA test when baby is first born to rule him out as the father (secretly), without upsetting your OH and potentially damaging your relationship? I no it sounds sneaky but I personally would try and find out without OH knowing. Hopefully after your dating scan you might have a clearer picture of what's going on!
Good luck to you my love xxxxxx