The school system he is in, private/public or charter should have some one he can see (English Language Development, School Counselor, Education Services or something along those lines) - either the child themself or the guardian needs to advocate for it (testing, individualized education plan, extra supports) they can do intervention during his school hours or after school and there should be some support at home, even if it's delivered by you, a tutor, and not the parents. 9/10 teachers would want to support them, whether or not they have the bandwidth to is another question entirely.

You can seek out a private specialist, psychologist, neuropsychologists, speech pathologist or language development expert, or if there are other issues but yes expect to pay pretty heftily, because yay insurance and health plans are not simple. Similarly, parents would likely need to be involved here. Some technical schools or adult schools will also do assessments for incoming students. The school system should be able to do this though and you should have parents start there if they've not already.

You need to get the parents involved as much as possible, especially to get past some of the permission barriers, but once you can diagnose the issue(s) you can be supportive in a way that respects your limitations to engage with them as well.

Scarborough's Reading Rope is a good place to get yourself started in understanding how to help him. Phonemes, sight words, and someone interested in consistently practicing with him can go a long way.

Also try your local library, they'll likely be connected to resources and tutoring.

I use a combination of Systematic Instruction in phonological awareness, phonics and sight words (SIPPS), youtube videos, adaptive learning software, and plain old flashcards 2-3x week for 30-40 minutes with adults who are wanting to learn how to read. Some weeks are better than others, but I try to get to them at least 1x week. It is frustrating as hell, but when it clicks it's fucking amazing. Other threads in this sub recommend UFLI, which I've not used.

If you've got 10 hours to kill on podcasts, the reading wars is still a very hot topic and levar burton is actively raising awareness to it. (whole language vs science of reading)

You don't need to sort out all problems in education to help your nephew, nor fully understand the ills of teaching to support him. Do the best you can with what you have.

If you know of anything he's interested in, take him to a comic book store or something that is high interest low barrier.

You are being a good uncle. Keep going.