You are obviously absolutely sure he is the dad, but he can't be, so it looks as if you will have to go for the DNA test, if only for your son's sake. You will have to also acknowledge that id the CSA are involved in that, forcing hom to take the test at his own expense, they are unlikely to let him walk away "footloose and fancy free", which will definitely have an impact on his marriage.
You are really in a bit of a cleft stick, since even if you don't want to cause him trouble, it has probably now gone too far for it not to do so, so you may have to accept that whatever happens is going to happen and be prepared to take the flak and the pressure which will result.
I am sorry , but I cannot see an easy way out of this, one official bodies get involved, they are like an unstoppable steamroller, so I think you just need to be ready for the result. I really wish you the best of luck, because this clearly was not what you wanted to happen. I hope I am wrong - please do let us know the result!
All the best, A.