This suggestion is going to sound cheesy but I must say take it easy (Cambridge Academic Content Dictionary). It taken to mean please relax, as an imperative suggestion to remain calm depending on the context, making it easy to incorporate into the vocabulary. It is also nice because it is almost as short as the phrase it's replacing, "Calm your tits!" which is only one syllable shorter.
It also has special signification to the admirers of the japanese computer game series known as Touhou Project by Zun. Although the original games are fairly innocent, it should be forewarned that some of the admirers are immature, vulgar, derisive and talented people, who sometimes create obscene art. You would probably need to supervise his direct access to Touhou subject matter if he became interesting in it.
However, although sometimes loathers make it gruesome, this is generally one of the overall more innocent fads amongst enthusiasts:
" The Yukkuri shiteitte ne!!! (ゆっくりしていってね!!!, often translated as "You should be taking it easy!!!" or "Take it easy") is a very common meme amongst the Touhou Project fandom, which involve giant, disfigured heads resembling those of Touhou characters, often referred to as "yukkuris". It originated on 2ch on October 13, 2007, when an anonymous user tried to replicate Shift JIS art of Reimu Hakurei and Marisa Kirisame, but failed horribly. However, another anonymous user thought the failed attempt was hilariously humorous and responded by drawing the infamous original picture. " [pics. omitted]
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Enthusiasts have much folklore regarding these amorphous heads and their legendarily irritating laziness. It has in ways superseded the popularity of their source materials
The relevance of this is that since this is related to a fad that's very popular amongst Japanese animation lovers, it should be relatively easy to get your son to pick up the phrase with amateur made "study materials" that might catch his interest. Well maybe, it's harder to find fully translated stuff. You'll want to download the pictures since some of the other pictures on Safebooru might be a inappropriate for a 12 year old. Limiting myself to pictures with the idiom of interest I can find:
Also this is probably not related to Yukkuris, except through coincidence but Hatsune Miku (Have you heard of her?) knows how to take it easy, although that is mostly in Japanese.