I understand why you would feel this way, but personally, I think it's not the best course of action. Also, all the advice I've read on this subject recommends against it.
Why does it matter if he is conceived through donor sperm? It won't make him less loved; it won't mean he belongs less to your husband or that your husband is less his father. There is no way that that knowledge needs to negate or reduce the bond you have with him.
So why shouldn't he know?
The positive side of him knowing is that then, there are no surprises. If for some medical reason the fact comes up, it is simply a fact - not a surprise or a shock.
I would worry that if he finds out unexpectedly, he might feel betrayed or deceived.
My DD is conceived with donor sperm. We have no option except to be open (we're both women), but I have found that many people confide in us that their children were conceived via donor sperm or eggs. It's not that uncommon, but I think people often don't mention it spontaneously.
Personally, I'm quite interested in it all and am enjoying seeing shadows of someone I don't recognise in DD's face. I also slightly hope she feels able to make contact with him at 18, so we can say thank you.
DD's not biologically mine, but it couldn't matter less.