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The clear looking fluid with little or no odor is probably unconcentrated urine. The body recycles the water out of urine usually only leaving a more concentrated solutions of wastes to be passed as urine.
This may be a sign he has a bladder issue such as an infection or stones. It could be he has a kidney problem. But it might be he just isn't holding the sphincter valve shut well when he is asleep too.
I would plan a vet check up to see what the problem is.
If he is 'leaky' there may be meds to help or you could try a belly band with a stick on incontinence pad to make clean up easier. You can see belly bands here
Hope this helps you!
You said he is leaking overnight that is what I meant.
Yes he is aging and problems start sometime and somewhere to first show up.
I'd suggest a vet visit to start investigating the problem - might be that meds will clear it right up.
It could be a pinched nerve in his spine is causing the bladder control issue but as long as he is acting OK and eating and drinking OK etc. he is not likely in pain.