help..

So me and my SO have been together Over a year and when we first got together three years ago ( we've been together twice) he suffered from drug problems, it made him real depressed , suicidal and he turned into a d*ck towards everyone including me, he was reckless . That's why we broke up the first time because it wasn't what I wanted to take to meet my mother and he didn't treat me very nice. However we've been back together over a year now after him being sober for a good while and he proved himself to me and it's been the best year of my life he's treat me like a queen , off drugs not took anything at all. However the other day we was at a party and I caught him sniffing MDMA which just broke my heart to see , I stormed out crying and we spoke after he was sober again he said he felt 'pressured' as all his friends were on it and told him to, like nagging him. I've been non stop thinking about it , he's had a rough time lately ( his aunt passed) and I'm just trying to understand why after everything he's been through with drug use .. What do I do? He promised me he'd never do it again when we first got back together I don't want him going back down that road, I love him to bits :(