Why is my dude bald?

My girlfriend has this lovely 8 year old boy cat called Norman. Hes an all black outdoor cat that has full acsess to a garden, our house, the shed and thats about it. Other cats come in the garden sometimes but generally he doesn't leave the garden if he's out. Big guy recently has this bald patch near his back right leg. Its about 2 inches with various amounts of hair loss doesnt look shaved. Theres no irritation or wound, so i dont think its from a fight or its been ripped out.

He has a sister who hes bonded with, and a younger sibling from a different litter who hes lived with all his life. Recently, we adopted 2 kittens (about 3 months ago). He doesnt like them very much but hes learned to live with them. He doesnt attack them, doesnt fight them, etc. Occasionally hisses to assert boundaries and we havent stopped him doing that.

Otherwise, hes the same happy and playful self. He loves cuddling with us, hes been eating and drinking well, not hiding, etc. Is this just a random thing? Hes not balding anywhere else, too