When the uncle tok him in, he probably explained to him that he was now his son. Hapo ndio makosa hutokea. If you take in a kid, you have to seat him down and have “the talk”. Explain to him you are not his father, and that you are trying to take care of him as an uncle, set boundaries, and explain very clearly his parent are gone. If possible, take him to therapy so that he understands that you are not his replacement parents, you are just his uncle taking on responsibility so he does not end up on the streets.
Do not let things assumke themselves. That is why most kids raised by family end up developing a seemingly ungrateful attitude. Do not expect him to automatically understand the gravity of the situation. Expect him to behave like any child who has limited understanding. Also do not attempt to pretend to be his parent, he will hate you more for trying to be his parent