Hmmm. I don't understand why you'd have to talk your husband into it. Would it make a difference to him if it was a girl doll or a boy doll?
My son was raised with an older sister and he loved playing with her dolls. I think it's perfectly normal for ALL little ones to be fascinated by babies and therefore play with dolls.
Little kids this age are totally "generic" and they are figuring out the world around them.
Babies? Everyone loves babies. We hold them and pat them and kiss them. Your son is certainly figuring out that he's not the only baby in the world. Having a doll and engaging in imaginative play is actually quite healthy.
When my son was that age, he wanted hair pretties in his hair because mommy and sissy had them. My husband was practically an ape and surprisingly, it didn't bother him one bit. My son played with dolls, had his own toy vacuum and kitchen sets. He's a junior in high school now and definitely all boy. However, he still loves babies. He adores my grandson, (his nephew). He holds, him, changes him, plays with him, feeds him. He can't wait for him to get big enough to teach him how to fish.
I would let your son have a baby doll. They make some that look like newborns that you can put any type of clothes on, girl or boy. The kid is kissing a diaper cream box, for heaven's sake, lol.
Just my opinion and best wishes.