Don't make drugs out to be this un-escapeable boogeyman it isn't. If he wasn't on stuff like heroin, which has heavy withdrawal symptoms, then not doing drugs for 2 weeks just isn't a hard thing to do, provided that you also change your setting/context. 

You do drugs, in general, because you feel better on drugs than not on drugs in the situation/context you're in when taking them. Being on vacation with someone you care about feels good and he knew that being there on drugs would feel worse since OP doesn't approve of drugs. But the moment he was back in the old context of them not living together and work and all that then being high was preferable to sober. This applies to many drugs like weed, coke, amphetamine, mdma, lsd etc.