Low functioning autism?

I suspect that my nephew has low functioning since he is nonverbal, has trouble doing things like tying his shoes, navigating from his school to home, remembering phone numbers, remembering his address, picking clothes out, showering (he spends about 1 hour showering alone). He does stim and fidget, very picky on what he eats, behaviour issues and sensory issues. There is times he can actually get verbal but is mostly nonverbal. As he’s grown his behaviour improved a little bit. He’s very bery intelligent. He’s 12 now but can easily do multiplication, addition, subtraction, Division, graphs, equations. Where could he go to get tested and where could I get information? (His parents are drug addicts that paw him no attention so he stays with me a lot)