HAs anyone been in this situation... and what should i do?

Okay ill give you the story short... My fieonce and I have been together for 2 years, he had an addiction to Oxy(he use to snort them) he is 42 days sober he went to rehab, i just found out about the addiction about 3 months ago (he did it for about 6 months) Well he went to rehab (because i am 6 months pregnant with an amazing baby boy) but now he is sober. Last night though he ALMOST relapsed, he lied about getting his "snowboard" back from the guy who he sold it to, who is a druug dealer... when i found out him and i talked and now im just really pissed. Why would he want to relapse??? I have never touched a drug in my life so im so confused... i always am wondering is he lieing to me... ect. what should i do? this is such a mess.