Has Simba had a check-up recently with blood work and a urinalysis?
What does he eat?
Arthritis might be a possibility, as something like 80-90% of cats over the age of 12 have it to some degree. Would definitely be a good idea to mention this to Simba's vet if you haven't already.
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler