Lila,
were glad youre here!!! this is a hard time it sounds like, and i am sending hugs and good thoughts your way.
the only thing that sounds just a bit off here , (and i dont mean to be mean or harsh or rude) is that he has changed and its only been two weeks. he could change again in another 2 weeks. its too early to know if he has made a substancial thange, a change long enough to get out of rehab, a life change, or a 6 month change. no one will know.
when you say this is the second time you have been blinded by his heroine use... between that, and the fact that he is in rehab... and that it is heroine, a very addictive drug... make it seem like maybe he has a serious problem. so the fact that he gave you an ultimatem might be small in comparision to some other relationship glitches here.
in regard to him being in rehab and getting to talk at his discretion... he probably doesnt even get to talk when he wants to- its at his rehab's discretion so bare with him atleast there.
if your family has a history of alcohol and drug use, and with your BF in rehab, i think you may want to consider going to al anon or nar anon. you probably already have, but you might want to research addiction, and heroine addiction.
keep posting and stay strong!!!!