They’ve said the lawyer said it was a problem to try adoption and that the conservatorship was the way to go

Because a simple Google search for state laws will tell you otherwise. Because of his age there was no restriction on him being adopted. It was simple paper work that, IIRC, you dont even have to go to a court hearing for. The paper just gets filed and thats that. The conservatorship was actually more of a process that adopting him would have been.