one baby left

So glad you have one little one left and dad is sticking around to raise him. At least we dont have that problem in the UK snakes thats is! Yikes  If dad is feeding regularly and taking poop away then I wouldnt be too worried, my chicks only came to the nest box entrance hole on a day or two before fledging, Im sure he will start peeping through soon as the time gets nearer to him fledging . Goodluck to him, have you any photos be nice to see.
You can buy tiny cameras to put inside the bird box and wire it to the tv or laptop, a lot of us do this on here, they are quite cheap too its a great way of keeping an eye on things.