If he rubbed his finger deeply into a cold sore and then touched the mouth or genitals of your child and rubbed it in well, transmission is possible. but how likely is that to happen? There is no risk of a the child touching his face. You overestimate the risk of transmission I believe, by quite a bit. Perhaps, if you do stay together, he could consider daily antiviral therapy to reduce the frequency of his cold sores and help to put your mind at ease.