I have a friend whose husband didn't know he was adopted.
For years, he listed family medical history that wasn't accurate - including when he had his wife were planning children, because she was a carrier for CF. According to his 'family history' he wasn't.
They got lucky with their eldest child. They didn't with their second.
His mum died last year. They hadn't spoken in fifteen years - and you can't blame him.
For medical reasons alone, OP, you have to tell your child. Yes, the donor clinic will have taken a full history, and will probably have ruled out anything as obvious as CF, but it was only accurate to the day it was taken. Chances are your donor was a young man - cancer, diabetes, cholesterol, heart disease are all genetically linked, and all will only, usually, start to show symptoms later in life.
Too, what happens if your husband is diagnosed with something with a link. That could lead to assumptions being made about your son's health in the future. A solid history is vital in modern medicine - you'll be lying every time you take him to the GP!