princess - so, when he is unwell and it turns out to have a genetic link, only neither you nor dh have the gene.
Or when he is getting married and his wife says she is a carrier for X gene, and is he, should they get genetic testing because of future kids, and he says no, because you neither you nor dh are, but maybe his biodad is?
Or when he fills in ANY medical form and is asked - is there a risk of heart disease (or whatever)
He may never know if you don't tell him, but I actually think it is really very unfair to him to deny him this knowledge.
As others have said, there are great books about. My kids at this age started to ask about where babies come from, you just include simply phrases like 'sometimes mummys and daddys get help from someone else to make a baby.' It is part of his story, and Dad remains firmly Dad.
Finding out as an adult will destroy his relationship with his Dad, finding out now is a non issue.