Out of curiosity, did you get a black one or was part of it in a bright colour? (Legally it should be half painted red, green or blue)
Its likely a problem with the low quality charger and battery but as Roger said, it may not be worth investing more money into that purchase unless you can try a different battery.
How are you at soldering? If you can do basic soldering to change the plugs then its worth getting a Li-Po battery and charger to try because you'll also be able to use them with any other rifles you buy (assuming they fit)
If your son really likes AKs I would recommend this one as a starter.
https://bbguns4less.co.uk/products/cyma-cm522-ak47-with-full-stock-in-blue.html
There would normally be other cheaper options but unfortunately there's a national stock shortage at the moment which will likely last into the next year.
Is he set on an AK type?
THIS or THIS are the cheapest available alternatives.
Or a spring sniper rifle which doesn't need batteries at all. https://www.patrolbase.co.uk/airsoft-two-tone-sniper-rifles
Pulling the cocking handle back does take some effort so only really suitable for ages 12 and up I would guess.
(Make sure to select a 'Two Tone' colour option when ordering)