It’s great that he finally wants to change, that’s the first major step to anyone’s recovery. With your help and supervision he could get better with help from the gp aswell. I don’t know how heavy of a user he is, but if he can get through his withdrawl stages, which will be agonising for both of you, the gp should be able to offer you support with subutex or methodone, but from experience with my partner the subutex are the best option, as methadone is very addictive in itself. To help with the stomach cramps and other withdrawl symptoms try buscopan and other remedies for the chemist, these will ease the pain. Seeing him in pain and knowing that the heroin will make him feel better will be difficult but try and get through it together and support him and hopefully you will be able to get through it together.