I doubt dehydration would have been the cause.
Impaction is something that is fairly common in older boars, as they age they lose muscle tone in their anal area and their skin becomes looser so they can't push out the poops like they used to. This can particularly happen if they have some poops that are a little softer. The poops that they can't quite push out get stuck in the anal sac and build up into an impaction plug. Impaction in older boars is almost always a reoccurring issue so this is likely to happen again, and he may even start getting them more frequently as he keeps getting older, so you will need to keep an eye on his rear and clean him out as necessary.
Edit to add: If this does become a reoccurring issue you may find you sometimes find a somewhat green coloured kind of poop that has a strong smell to it in the impaction, these are the half digested cecal poops that they normally re-eat to digest a second time. If he doesn't get enough of these half digested poops his health may start to decline so if you find them stuck in his sac offer them to him so he still has a chance to re-eat them.
