They say not to cut down on fluids. It actually could be the temp drop that happens round that time. Have you thought about getting your DH to pop an extra blanket on him at 1.30 and seeing if that makes a difference??
At this age they're abit like puppies. Puppies can hold on for an hour for every month of their lives and TBH at just 3 they're still developing control over their bodies. My mum says (coz shes sitting next to me so I asked her

) stick with it and you'll likely find that over the next few months, specially with the warmer summer weather he should get the hang of it!