Based n what you know or surmise I'd still go with the Thai granite. The Japanese ridged one for sesame seeds is a specialty thing, and salsas and guacamole don't need ridges. It is really more your upper body and the pushing and sliding motion that creates pastes or blends Unless working with hard ingredients to make spice pastes like Robin describes here https://eatingasia.typepad.com/eatingasia/2008/04/the-balinese-da.html - in my opinion you don't need roughed up surface. The inside of granite one not hy*per smooth, you can feel rough on your finger but not ridges. not a big ticket item so I would just get him a general purpose one and let him play. I can't reach mine (stuck behind lawn tractor w flat tire) but like this one but mine is taller - one in middle https://forums.egullet.org/topic/19519-mortar-and-pestle-–-the-topic/?do=findComment&comment=2073443
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