If my bf had syndactyly, what is the possibility our kids have it?

Depends. Syndactyly, or fusion of the fingers or toes is one of the most common limb malformations & nearly always an inherited condition. Unless your bf knows exactly which pattern of inheritance his is, can't tell 4 future.Its a minor & fairly easily corrected malformation which occurs in otherwise healthy kids. This is not a result of something mom or dad "did'! http://www.Ncbi.Nlm.Nih.Gov/pubmed/223339.