How can a captured druid be restrained so that he can't shapeshift to something that can slip his bonds (such as a small snake)? I would prefer minimum or no magic used. And what about natural-fibre ropes—if those count as "worn" items and simply melt into the druid's form when he shapeshifts, is there any way to prevent that?

The Druid in question is level 7 and has been captured by nomadic barbarians with only limited access to magic. They know he's a druid and know that druids can shapeshift. Tight rooms are out of question and the barbarians can't afford to use high level characters as guards, at least not all the time.

Clarification: I seek answer that is also believable, not only exploitation of rules.

I didn't think that reason why druid is captured is important, but I add short explanation:

Barbarians are seeking certain holy locations and they think druid knows them, so they need him as guide. They didn't plan to capture druid but managed to surprise him after he escaped from disastrous battle. (This is side plot which should give players hint about bigger picture).

I am not looking for a strict RAW answer, but a believable solution.