If you go for adoption, when that child is placed in your DH's arms, he will be DAD, in capital letters. There will be grief to work through as you both let go of the biological process, for sure. But the joy of your new baby will be incredible.
My DH was choked up the other night with tears in his eyes when DD (who is one of our adopted and now-grown daughters) called and talked to him on the phone and asked for his Dad advice about a guy she was interested in. He was so proud of her and they had the sweetest conversation, as they usually do. I don't think he could be more emotionally connected to another human being. Biology and DNA is irrelevant at times like this. This DD, and the other kids, have grounded him, centered him, inspired him, and connected him to everything that is important. The conversation could have been her, or one of her siblings. He is Dad to all of them, no matter how they arrived in our family or where they were born, and he loves it.
FWIW, this DD is not quite 5' tall and 90 pounds soaking wet. She has black hair, dark brown skin, and was born 8000 miles away. DH is pasty white, 6'3, overweight, and gray hair.

They are father and daughter, through and through.
It sounds like your DH would be an incredible Dad and he wants that experience. Please consider the journey to parenthood through adoption.