A question regarding renewal of postdoc fellowship, and the mentor not making the picture clear
I find "I'd first like to work with you and see how you do" perfectly reasonable and that's probably his actual position on this issue. What he said during the interview ("depend on the results of his next grant application") is probably also true, just not the whole truth. He wanted you to accept the position after all. As you know, until a contract is signed you have nothing. 

Try setting up regular feedback meetings so that you know "how you do" and can try to improve your performance (and his perception of it) and can adjust your search for the next position accordingly.