Hi Jobey68,
What a shock for you, it takes time for you to get used to the situation and your own feelings before you can do anything. OCD can produce some really unpleasant thoughts and your Son must be relieved that he has a diagnosis. Your DIL must also be getting used to the situation in her own way, not easy when you also have a baby to deal with.
Both of my Son's have OCD and it has been a struggle, but there is always light at the end of the tunnel. One is married with a lovely child and the other is in a stable relationship, both of their respective partners are fully supportive and life goes on...we all rally with support if needed. The best thing I can advise is to get to know the illness and try to understand it and the type of OCD your Son has, that way it is easier to support, and to know you cannot cure it for him. Listening is a big help, but sometimes they do not want to talk and that is okay too.
Maybe your DIL can educate herself in time to support him, but it is important to also seek professional help for your Son,
Early days and baby steps, hopefully your grandchild will offer you all some comfort.
Wishing you all the very best....