I am sorry to hear Simba is lethargic, not eating and drinking well, licking his leg and trying to hide it from you when you try to look at it.
It sounds like he may have an abscess on that leg from an accidental bite or scratch when playing with your cousin's kitties.
Generally these occur secondary to a bite or scratch wound that injects bacteria deep into tissue. The original wound may be so small we never notice it and heals quickly. Meanwhile the bacteria deep in the tissue proliferate and lead to discomfort and inflammation, thus his licking, and eventually the tissue in the area starts to die from the inflammation, pressure and bacteria. When that happens the hair over the area may fall out and an open draining wound can develop. The entire process usually takes about a week to ten days. They seem to blow up overnight once the infection gets established.
Treatment is based upon draining the abscess fully, removing the dead, infected tissue, and starting an oral antibiotic to get rid of the infection.
Since he's not eating and drinking normally and seems very uncomfortable this does warrant a visit to his veterinarian as soon as possible.
In the meantime, if he will let you, you can warm compress the area he is licking with a clean cloth moistened with very warm/hot water for 10 minutes at a time several times a day. This will hopefully cause the abscess to drain which would be best for him.
If the abscess opens up and he will let you flush the wound with a mix of betadine solution (also called povidine iodine solution and available at the pharmacy over the counter) and warm water (mixed in a syringe so it looks like weak tea), a couple times a day for a week to10 days to keep it open and draining. It may heal from the inside out with cleaning and flushing if he lets you do a thorough cleaning and flushing. Do not let it close prematurely or the abscess will reform.
In most cases though topical treatment doesn't work and they must be seen by a veterinarian and get oral antibiotics prescribed.
Please let me know if you have any further questions.