DO NOT DO IT!!! Repeat: DO NOT DO IT!!
Repeat: DO NOT DO IT!!
First of all, he cannot "sign you out of the loan." Can't do it, at least not without the lender's approval. Think about it: You agree to loan Joe money, but only if Bob agrees to co-sign. So, how would it be fair to you if Joe can just sign Bob out after six months. Therefore, the terms of any loan cannot be modified without everyone's approval, particularly the lender's approval.
Next, there is a good reason why he needs a co-signer: It's because his credit is crap. If he can't make payments on the mortgage, then YOU will be the one who will be legally responsible for the missed payments.
Here's how to handle this: Tell your uncle that you and your fiance are shopping around for a house of your own, and for your "future" family, and you cannot afford additional loan obligations on your credit report. If your uncle is not a jerk, he will understand. If he continues to pressure you, then he is an uncle that you really don't need.