Cant remeber this film?

i need help to remember a film title. Well theres a boy and hes shrunk by these people under ground and he has to get something but theres a bad guy. He also lives with his gran and there a guy going to kick or buy there house and theyll have to move also in the end the boy finds a Diamond and he pays for the house to keep it or something. also when the boy was small he had crazy white hair. does anyone know what this film is called.