Your ex is just trying to protect his half of the interest in the property.  An action for partition is just his way to obtain an order of the court confirming that he owns one half as an individual and not jointly with any rights of survivorship, as I am sure that the deed states if you have never had it changed since you bought the house when you got married.  Even though the divorce decree may have divide the interest, the actual deed did not change.  There may be no point in fighting the partition action if it is really just an affirmation of your divorce decree.  And although you are doing everything pro se it may be a good idea to at least consult with an attorney on the matters at hand and see if there is anything you can do to enter an appearance and consent to partition.  Saves time and energy.  Good luck.