
Hi Emma,
This is quite common in young pups. When he is lying on his back in your arms having a cuddle (I know what we're like with puppies! ;) ) can you feel any hint of the other testicle nearby? If you gently stroke in the right direction late descenders can sometimes be encouraged further on their way.
Otherwise it's a question of having to wait and see. If it's not down by the time he's about a year old then he may need an operation to remove the retained one. But there's
loads of time before you need worry about that!
Good luck with him.
:)