Could have been a stroke or a heart problem causing his weakness. It might also be hip or spine pain impacting him.
Also if his eyes look odd to you it might be vestibular disease causing his disorientation and distress. You can read about this inner ear problem here
www.marvistavet.com/html/body_vestibular_disease.html
If you are able to keep him comfortable and help him out for potty trips etc (a towel under his chest behind his front legs can help you support him for a trip out) then you may be OK waiting to see your vet in the morning. If he seems to get worse calling the emergency vet and considering the trip might be your best bet.
Hope this helps you!
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