For Samuels adoption

My son Samuel (6) had ALL ( Acute lymphoma leukemia) . He loves my husband and calls him dad but always asked if he can have his last name, of a course my husband would love to give him his last name. So we been looking around to see if my husband can adopt him for his 7th bday in September to surprise him. We were told it would cost us anywhere between 2,000-4,000. We can't afford it . And would appreciate any help