It is true that there is a possible risk to the fetus if a woman is infected with Parvovirus during the pregnancy, however, this risk is very, very low. First of all, this risk is higher in the first half of the pregnancy than in the second half of the pregnancy. The other thing to keep in mind is that the majority of adults already have had Parvovirus or been exposed to it, and thus have developed antibodies to it and will not get it again. Thus more than half of pregnant women are not even susceptible to it.
Thus it is very unlikely that you will have any problem if your child does indeed end up having Fifth's disease. The other thing that you should know is that by the time the red rash on the face develops, the child is no longer contagious. Therefore it does not make sense to try and stay away from your son at this point or to prevent his exposure to other people. You should talk with your obstetrician in order to determine if she would like to do any further studies to verify that the baby is doing fine.