My husband use to tend to favor our youngest. Our girls are only 4 and 2 so we are not where you are. Our eldest is very serious, high maitenence, picky, fragile... Our youngest is not only his twin but she is so funny, easy going, silly and rough.
I brought up my concerns to my husband and although he didn't necessarily see what I saw he did agree to spend one on one time with both of them. He now has date night once a month with each of them. Because of these dates he has really come to love each of them individually. He loves taking my eldest to a fancy dinner and watching her act "fancy" and listening to her talk "fancy". He loves the way her little face lights up when he comes downstairs in a tie and how excited she gets to dress up. He loves taking my youngest to the park or an inflatable play place and rough housing with her. He loves to watch her get messy when she eats and listen to her grunt when she thinks something is yummy. It has made such a difference in their relationship. He really enjoys having two seperate relationships with both of them. They love their daddy like you would'nt believe. He is very well taken care of in this house of girls!
Maybe having your husband take each of your children somewhere one on one of their choosing he would see the great characteristics they all have. Since your children are older I'm guessing it will be a real learning time for your husband! It is so important to see your children as individuals not one unit or extentions of yourselves. Just remember not to fight with him about this or put him down. Maybe even talk to your kids first and have them go to their dad and say "hey dad this weekend could just you and I go..."
I hope your situation gets better.