How old is he? If I were you, I'd get him started right away. I saw you mentioned second language classes, I think that sounds great!
I took classes of French for many years in school, but it wasn't until I moved to France that it really took off. I know though, that if I would had been reading books and listening to podcasts etc prior to my arrival in France, I would have spoken a lot better.
A teacher is great because they give you explanations, feedback and corrections. But to really develop, you need to read, you need to listen to content that you are genuinely interested in. And just the content in school is usually not enough.
I would really recommend that he gets a lot of comprehensible input. He can for example start reading comic books like tintin (Tim und Struppi, see link below to Swedish amazon) and Dragon Ball. Comic books have the advantages that they are action packed, and if there are words you don't get, you can follow the plot anyways, thanks to the pictures.
I'd also start out by watching Easygerman https://www.youtube.com/c/EasyGerman (I'd watch 20-30 minutes per day). They have videos in clearly spoken german about everyday topics. You also have subtitles in both English and German.
If you have a streaming service like netflix, he could watch german content, or content like cartoons/anime in german. https://www.languagereactor.com/ This plugin to chrome allows you to have subtitles in two languages at the same time.