I think you have to let him know how much you appreciate his help, but also remind him that you have ideas too. When I am dealing with my mother, I always say things like...."Consider this"...and offer what I think might work. Sometimes that softens my wanting to do something differently.
Maybe do him the same favor, considering what he says.
Just guessing here....you are undertaking a large project, sure want you to be happy with the results. ....I just reread this and it makes me sound like I know what I am talking about. I don't! Promise. Ask my husband, who knows everything.
But when it comes to my sewing room, I do stand up for what I want. I'll say things like, "I understand what you are saying, but that isn't what I want in my sewing room." Really, I have actually said that.
Dina