Outside/ inside cat

Need help converting inside/outside cat to inside cat.My sister passed away
recently and she had always allowed her cat to come and go as he pleased (5yr
old neutered male).I brought him to my home and tried to keep him strickly
indoors but he just wanders around howling constantly.Finally(2 days later) had
to put him in the basement to get some peace but don't want to keep him there
long term.His behaivor is not a suprise as my sister tried to convert him
several times with the same result.Is cold turkey the best way or has anyone
found a better solution.
Al