My daughter and son-in-law have a king size bed, but it is split in the center with a king size bottom and two twins on top. Their 'twins mattresses' are each made up like you would twin beds. I am making them each a quilt, the same design, but in their favorite colors with the borders the same color on both of them to tie them in together. They love the idea of the quilts being the same design and with them being different colors.
So, I say go for his favorite colors, with the same design. That way, they match, but are each their own!