What do I need to do to complete legal guardianship for our son, and is there such a thing as co-guardianship?

Who do you want to become the guardian(s)? When do you want a guardianship to begin? Is the goal now, while the child is still a minor, or to begin upon his attaining age 18? Will the child, upon becoming an adult, need someone making all decisions on his behalf, or only need protection financially, such as having a representative payee for his financial affairs. Sitting down now with counsel is a good plan, even if there turns out not to be a need to act right away.