I wouldn't recommend incorporating until you need to. Unless you have money or legal documents that require a separate legal entity there is no reason to incorporate while you just have an idea. Incorporating is a bit more time consuming than filling in the check and mailing. Depending on the type of incorporation and the state you might find incorporating to be a serious distraction you can do without for now.

On the other hand if the reason your co-founder doesn't want to incorporate is because it would be a waste of $135 dollars than you seriously need to talk to him about his commitment to this.