Pet for the Little Boy

My little boy loves any animal. He chase birds in the park. He would sit on his "bumbo" for several minutes while watching the birds feeding in our backyard. He loves our cat, Lucky. It can quite a whiney boy. He loves him that I am looking forward of him leashing Lucky so he can tow him whenever he goes, just like a dog. Of course, it crossed my mind that we might end up having another pet at home the way he adore animals. But, it will be within Mummy's rules. Not high maintenance like horse (that needs stable, feeds, horse supplements, not to mention the VET cost). Do not litter anywhere - should not require too much effort to stay clean. But if the little boy volunteers to do all the job, I couldn't say NO, I'm sure. :)