Well that's kind of vague. Are you speaking of BDSM aggressive? If so The New Topping Book by Dossie Easton would probably be suggested to be helpful. If not, you can go to amazon
in that human sexuality section there, and pick out a couple of books to start from.
You might have to have a few sessions in the bedroom where YOU do the work of telling him EXACTLY what to do the entire time. After a few times doing that and seeing you enjoy it maybe that will help him take the lead on his own.
Me though, I'd probably just find somebody to date who likes to have sex that way. Are you sure he isn't content with how he has sex and isn't talking about finding his animal side because you're telling him you want him to be different? I mean all he has to do is google "how to be aggressive in bed" or something if he really wants suggestions.
I love sex with my husband but while encouraging him to get more aggressive resulted in a bit of change and the occasional wilder night, it didn't change enough that I don't crave that elsewhere on a more regular basis. Because I've stopped hoping for and talking about it with him, it has helped our relationship (including his comfort level surrounding this) by taking stress off of the subject.
I guess I think part of the point of poly for me is to not try to change my partners, but accept them for who they are. If I want something else on top of that I get to be lucky enough to be able to look for it elsewhere. I don't think many things are more destructive to a healthy sex life than to regularly be letting a partner know you wish they would do things differently or better.
So with that said, why don't you try being the aggressor and being specific and forward and provide positive reinforcement when you like what he's doing in response to your requests. Really, I'd do that straight for a few weeks, and if that didn't kick start him into finding it was for him, I'd just accept it wasn't going to happen.