You could totally do inspirational quotes from men about feminism for Jackson's room.
ETA: Joking aside, I do think there are plenty of values you want to instill in your son that society doesn't think is important for boys to develop and there are lots of quotes to that end. It doesn't have to be about rah rah inspiration to make an impact on your child.
Quotes on equality, fairness, compassion, kindness, sensitivity, etc would be good things to put in a little boy's head.
What about the idea of just including the whole family in the brainstorming of the women you will include? Sometimes the value lies in the discussion as much as in the action. Challenge the males to find equal examples of men and women to show them how often history is written from a male's perspective. I think including the males in the family to help with the wall will make a much larger impact for everyone.
I think you could do a couple quotes for Jackson. Make them about being a gentlemen and not needing to be a superman or have a certain body type.
As for your DH, women ARE seen as less, and DO need more help from their families and communities. And not to be a Debbie downer but parents probably reinforce certain inequities whether we realize it or not. So I guess I disagree with him but in a different way.