How long is his morning nap? You might want to either shorten his morning nap (usually an hour should be sufficient) or actually move the afternoon nap up, instead of back. Sounds to me like he may be overtired when he's going down for his afternoon nap, and maybe you're missing the "window of opportunity."
Also, keep in mind that he's suddenly at an age where he's much more aware of what is going on around him. He suddenly realizes that there is a world going on around him, and he just doesn't want to miss it. My DD went through this around the same time -- she doesn't like to miss out on anything. I actually put a book and a couple toys in her bed with her, so she had something to "wind down" with, if that makes sense. Some days she would take awhile to fall asleep, but she was usually content to play in her crib for a short period of time, alone.
It takes time, once they are aware of their own desires (ie "I don't want to nap!") to learn how to self-soothe again. If you need to nurse to get him to nap, do it unapologetically. but try to introduce something else into the routine that means "naptime." Maybe a certain book, or a certain song you sing. That may help.