Is he exclusively breastfed or is he on forumla, or a combination?
If his poo was hard/nuggety id guess he is having some formula.

The reason I ask is that its not unheard of for exclusively b/f bubs to go 10 days without a poo, and not be constipated.

If he is having formula it may be related, I would take him to the GP for some advice, either way if you are concerned.