Grayson is spiritually the bottom.
*Spoiler alert*
Actually in the novel Grayson asked Dane to top him but Dane refused because he was worried Grayson may like it too much and cling to him even more.
This was before they got together though.
After they got together, Dane couldn't stop thinking about tying him up and spanking him, although he didn't do it but it's implied that he would have certainly done it in the future and Grayson is more than willing to accept anything Dane does to him.
Dane doesn't keep him on a leash for nothing, he really became his dog

i was so hoping that greyson would be the bottom until i saw that dane is a dominant omega