If you've captured only the druid, there's not much you can do to stop him wildshaping and getting away. If you have (at least one of) his friends, threatening to kill them slowly and painfully if he tries to escape should be a good deterrent.
It's not explicitly spelled out in the book as to how size changes affect wildshape when one is tied up. One COULD try placing an inwardly-spiked collar around the druid's neck (so that a size increase causes the spikes to stab into the neck) and putting tension on legs + arms (as per a rack). Or if one is DM merely rationalize that confining a druid in that manner is effective at blocking wildshape.
In which case the tension on the druid's limbs pulls them apart as they shorten...depending on how the DM reasons that the change occurs.The druid just wild shapes into a very tiny animal, or if it is a worn item assumes it into his form.
In which case the tension on the druid's limbs pulls them apart as they shorten...depending on how the DM reasons that the change occurs.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.