Whether he did or didn't , I don't see how he could have known it was going to happen again. Most cases are not inherited. And abnormality usually occurs in egg cells (meaning it comes from the mother). Pretty rate for it to be coming from the father.
"Most cases of Down syndrome are not inherited. When the condition is caused by trisomy 21, the chromosomal abnormality occurs as a random event during the formation of reproductive cells in a parent. The abnormality usually occurs in egg cells, but it occasionally occurs in sperm cells. An error in cell division called nondisjunction results in a reproductive cell with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of chromosome 21. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra chromosome 21 in each of the body's cells."
http://ghr.nlm.nih.gov/condition/down-syndrome