DH had a countdown going 'til 1 (since I'd told him that babies wean around 1 year of age, Mom said so and I didn't research much beyond that when Ina was little) - partially because he's a breast man and partially because I was on a really restrictive ED with Ina and he wanted me to be able to eat regular food again too.
But Ina's allergies were still there as she approached a year of age. I started researching "extended" breastfeeding because we were afraid she'd wean before there would be a safe, accessible (and cheap and delicious) calcium source to replace my milk ... was pleasantly surprised to read that as long as I didn't buy into some of the mainstream info about solids introduction etc., Ina would likely nurse 'til 2 or older. The more I researched it, the more sold I was on it. And as I researched and read up, I'd read things to dh too. And he became more supportive as he learned more too.
Ina was about 18 months when he laughed at his sister while talking on the phone with her, after she told him that Ina was too old to keep nursing and should be weaned. He thought she was kidding, since *everyone* knows that babies should nurse 'til 2 or older. (I wish!) .... She hung up on him, she was so mad at him.
Anyway, I think that it is very possible to help a dp move from "ewww, that's awfully old," to "Yes, it's good, but what will X/Y/Z think?" to "What's wrong with X,Y,Z for NOT bf their baby this long (or longer)?" That's the progression that dh has followed. He went from doing a blanket dance while we were NIP an infant, to walking calmly through a zoo while I carried a nursing 18 month old, and beyond. Any more, he's actually quite judgmental of people who wean at a year!
In some ways, he's a much more brutal and open lactivist about that sort of thing.
I think a large part of his change of heart (beyond the research and actually seeing it happen) is that he's seen how much it helped Ina as a toddler, to still be bf. When we were gently weaning during my pregnancy with SJ (nursing while pregnant was awful for me
I'd hoped to do CLW with Ina but it was not workable) - he would come and remind me how very much our 2 1/2 year old needed to nurse, and how important it was for her. He was very much an advocate for her at a time when she and I both needed that.
I love my dh!