1) deed of cancellation can be executed with consent of the parties
2) if your husband was suffering from dememntia how did he execute gift deed . ?
3) your husband is not of sound mind as he is suffering from dementia. and ALzhemiers .
4) if step son refuses your husband has to file suit to revoke gift deed on account of coercion or undue influence. however suit may take years to be disposed of .
5) contact a local lawyer