Due on sale clause is very unlikely as long as you are current on the payments. I have a lawyer I am happy to recommend for the contract.
With the appreciation we saw here, it is unlikely that your uncles house is worth less than the mortgage that is on it. In most cases, the seller retains the equity in some way.
For example:
Your uncle’s outstanding mortgage balance is 50k, but the house is worth 100k on the open market.
Your uncle would net: 100k - 6k (realtor commissions)- 50k (mortgage payoff) = 44k
Couple of options from here, you would need a down payment of 44k, get a new loan for the remaining 44k to pay your uncle, you can do subject to and have a balloon payment set for 2-5 years when you can qualify, or you uncle wants to be generous/doesn’t want to hassle with selling on the open market and you walk into 44k of equity.
Hopefully this makes sense. Happy to walk you through the process or answer anymore questions.