If you're worried you could ask your doc or Plunket nurse to feel his tummy (if he's consitpated they'll be able to feel little 'rocks' in there, and can probably show you what to feel for if you want to know for next time).  If he's done skids but no poos then I'd say he's consitpated (I'm not a doc though!). 

May sound weird, but if you have any pure lanolin for yourself, you could rub some onto the outside of his anus a couple of times each day so that it's a little less likely he'll get a fissure when he does eventually go.  Can help heal one too.  prune juice diluted is good - about one 10th juice to water.