Unfortunately, as the original application was dismissed on the basis you were already his guardians, just by virtue of being his parents, this status would normally expire upon your child reaching 18 years of age. As your child is now an adult you can only remain his legal guardian if apply to be appointed, or if he is legally competent to do so, by being appointed by your son. To apply to be appointed, if your son is not legally competent to do so, you need to apply to the State Administrative Tribunal, much as your husband did previously, and you can read about this process here:
Depending on your age, it may also be prudent for one of his siblings to also be appointed as an alternate or as a joint guardian.
The SAT process has only a notional cost and unless opposed is unlikely to be a difficult one.
I trust the above assists.
Good luck and please rate my answer.