This is not an adoption record.George W. Collins was appointed his guardian and as such he was responsible for seeing that his bills were paid from assets available to William H. Collins. If there had been no assets - or an insufficient amount - the child would have been apprenticed to learn a trade. Since he is listed as an orphan, we know his father was deceased, but he might not have died in Marshall County, TN. You might want to check neighboring counties to see if a Collins had died prior to the date of the guardianship record. Note that the document says he "now resides in the county of Marshall."
Also, if William's father had died some time before 1840, look for an earlier guardianship record whereby the county court was appointing the guardian. It was only at the age of 14 that a minor could choose his own guardian.