At 10 years old I would have some blood work done just to get a base of how his organs are doing and especially because you are not familiar with this cat. Cats are notorious for hiding pain and sometimes owners don't know anything is wrong until it gets severe. 
It sounds like he is dehydrated so I would ditch all kibble, nothing better for a kitty's urinary health than good flushings twice a day. My cats pee at least 2 times a day, normally 3.
This is probably one of the best websites on cat food:
http://www.catinfo.org/
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Cat maid to:
Rose semi feral, a cpietra rescue, female tabby (approx 13 yrs)
Jasper RIP (2001-2018)
Sweet Pea RIP (2004?-2014)
Puddles RIP (1996-2014)
Snowball RIP (1991-2005)
In a cat's eye, all things belong to cats.-English Proverb
“While we are free to choose our actions, we are not free to choose the consequences of our actions.” Stephen R. Covey
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