Hmmm... Looks like he is not too interested, maybe doesn't yet have his own reason to do it. By the way, look at these pages 👇, we have them also in Spanish. With others Bears and different motif as well 
https://www.bitcoin4youth.com/ellis-from-el-salvador-creative-pages-to-print-at-home-01/ Maybe he'll be interested in colouring, solving puzzles etc. Not directly learning English here but getting some words in between, once he gets interested it can get expanded with more /otter things. If he's too old for that, try a similar method - instead of making him learn English add for example once a week some activity that will make him use this language even just a bit. Maybe Sunday breakfast, whole family with a few words in English to be always exchanged, by everyone (naturally built habbit). Tuttle Twins move was a smart move 💜👏 Maybe add sth extra to it. But not to 'learn' English but do sth that will somehow include this language in it (Site note: I'm not trying to lecture you, just giving some inspiration ☺️). 

 He is indeed older than the target of those pages 😉 But it seems his interest has sparked again and his duolingo profile is progressing further, I keep him in my lap while he does the lessons, and he likes seeing his avatar climb the score list.
Considering how easy it is for english stuff to end up in front of him, I am confident that as soon as his vocabulary is large enough he will be even more motivated because he will notice it's a useful skill.
I am not at all opposed to videogames even, the old SCUMM adventures are visually pleasing and full of interesting clues