Okay, well I know messy husbands can be hard to deal with, I have my moments with mine too, but I have to just say that if you have an agreement that he does the outside and you do the inside, then I don't see that you have a huge argument here. I get that it's annoying that you're doing "your part" and you want him to clean up out there, but if you don't have to deal with it at all, then I guess that's good right?! Maybe a little bit?! ;)
Ultimately I understand why you want it cleaned but I'm not sure it will happen. Maybe you guys can talk about it and set up a plan to clean/put away certain parts or items each weekend. Have him pick one thing and maybe you pick another and agree that it has to be done by the end of the weekend and he can have all week to work on it or he can do it all in one day. If it continues I would probably just go ahead and hire someone to deal with it but not until you've talked about it with Hubby and tried again.