3yo w/severe autism

What are some learning things, tips/tricks that have made a difference on a child with autism. He’s very smart and loves numbers and the alphabet. But besides that he doesn’t talk or make sentences. He’s my sisters bfs child but is around often and just started headstart. His mom lives 4 hours away and they don’t want to work with the fact that he has a disability. They are African and believe that their kind don’t get things wrong like that even though it’s very clear. Even the doctors told them and they freaked out and cussed the doctors out for even saying anything. They’ve had him attached to a bottle and didn’t keep him on any schedule which didn’t help him but since he’s been around me and my sister have gotten him off the bottles and on a set schedule and he does better but he is still very severe