With us, DH is the one who brought it up first, and that is what we ended up doing to get pg with our son. In our case, DH has zero spermies, so we have absolutely no chance of having a biological child from both of us. Perhaps that is the difference in his reaction versus what others may be feeling. I don't know.
I hope you don't need to go this route, but if you do perhaps you can use what my DH & I researched and made our decision on in your conversation with DH. So for us, we ended up looking at it as me being the birth mother - and we would know ALL my family history and that I got excellent prenatal care for the entire pg. and with the donor as the birth father, we got alot more info then we would have with adoption.
We looked at both adoption and donor sperm. Either way, we knew that we would be parents and just need to take different roads then most to get there. With adoption, we'd found that you oftentimes don't get alot of information on the birth father, and sometimes very little on the mother. You also have inconsistencies in the amount of prenatal care the mother received during the pg too. Sometimes it was wonderful, othertimes not as much. When we started researching donor sperm, using a bank, we found that you end up with alot of information about the donor. This will vary depending on the bank you use, but you typically receive a detailed medical history form going back several generations as well as other relevant info (hobbies, traveling outside country, occupation, etc.).
And I can say that in this pregnancy, you would never know that my DH wasn't the biological father. He's been to practically every doc appt with me, he's loved my growing body - will rub my belly, sings & talks to Mark, and Mark is going to be named after his Dad. We're even talking about having 1 more child, and hope to use the same donor. DH has even put in his "wish" of having a girl next time. I reminded him there were no guarantees of that.
I can't predict the future obviously, and do sometimes worry about answering questions later on (we do plan on telling our children), what the child will look like, how he'll react with the news, etc., but I also know that this baby is loved by both of us very much and he'll never doubt that. I've also been in touch with other women who have used the same donor, and may even meet up with at least one of them who by coincidence lives not to far from us. Our child will have half siblings out in this world, and I admit it is a strange feeling, but overall I also feel it is a blessing too.