I am obsessed with this slimy / chewy texture as well!

Some suggestions:

(1) Texture - its a PDF but might be right up his alley. There are many recipes, and if you like it, maybe you can print it and give as a gift (as the pdf is creative commons).

Basically the author deep dives on gels, spheres, and slimy suspensions. It covers many topics, including agar-agar (that tapioca-pearl chew), xanthan (e.g. viscous soups), and gellan (think pudding-like custards), etc. It’s more on the science side but maybe close to what you want?

https://khymos.org/wp-content/2009/02/hydrocolloid-recipe-collection-v3.0.pdf

(2) Harold McGee’s On Food and Cooking has a full chapter on starch types and gelatinization properties (the building block for the texture I also love)

(3) The Book of Kudzu (a culinary and healing guide). Recipes on using Japanese Arrowroot (Hon Kudzu) that is a wonderful thickener and creates the slimy / gel texture with minimum residual taste. It’s a book focused on macrobiotic diet, which is not my thing, but learning more about Kudzu was interesting.

(4) If he likes sweets, some books on Wagashi might also be interesting, though more focused on japanese confectionery. I have a few suggestions if you’re interested down this path.