As an adult studying Spanish on my own, here’s what I’ve found especially helpful:
YouTube - the Super Simple Español channel - These are actually animated preschool songs in Spanish. It may be a hard sale for a middle schooler because they have 0 cool factor. They’re themed to teach kids basic concepts which would cover a lot of 1st year vocabulary (numbers, colors, animals, body parts, action verbs, etc.), with rhyme, rhythm, repetition, and animation. There are also cartoons, but those would probably be better later when his Spanidh is stronger.
https://m.youtube.com/channel/UCyY3Wd5x85o8AKXjYSoxFAQ
Spanish verb book - 501 Spanish verbs
Duolingo - What I like best about Duolingo is the ability to repeat something as many times as I want, sometimes slowing it down. Personally, my ear for distinguishing sounds in general (musical, linguistic, etc.) is terrible. I like to close my eyes while the question is given, so if it’s an audio prompt, I can try to process it initially by the sound. Most questions have the Spanish audio available in the discussion, regardless. I’ll listen to the clip over and over until it starts to stick, then I try saying it, and when I think it’s sounding close and I’m getting up to speed, I say it while plating the audio do I can see where my pronunciation differs.
I also like the instant feedback. Instead of doing a set of homework and getting it corrected the next day (at best), you get immediate response for every question, hopefully before errors become engrained.
While Duolingo was always weak on grammar, and they’ve taken away the ability to post questions which is a tremendous loss, past discussions remain available and are invaluable. They contain explanations of common problems for novices, as well as more nuanced explanations by native speakers about regional language differences.
I think what’s helped me most in my Spanish studies in comparison to other languages I studied in high school and college classes was some advice I heard about how to approach language learning. They suggested that rather than approach it as an academic subject with content to be understood and memorized (although those are important), approach it like a physical activity that you practice until it becomes automatic.