Looks like a Japanese maple (Acer palmatum) with physiological leaf burn - unless the yellow speckling indicates a sucking pest (as in mite) infestation that is heavy enough to cause some browning. (You could also have both separate conditions present at the same time).

Look up "leaf scorch of maple", "leaf burn of maple" or similar terms to see if it appears to fit what you have, find out what is involved - Japanese maples can be prone to it. And get a magnifier and look at leaf undersides to see if there are tiny bugs such as mites. If these are found and you are in the US - it generally works best to indicate where specific plants are located when asking about them here - an independent garden center is likely to have suitable control products.

Finally if you are going to maintain this specimen as a bonsai you will have to start undertaking typical annual routines such as pruning the top and roots, replacing the potting soil. Already at this point, with its comparatively long and straight shoots it cannot be told in these two pictures that the plant is supposed to be a bonsai. And not an unaltered natural habit seedling or young grafted specimen.

Thank you for you insightful advice. It could certainly be caused by sun damage as I am in Texas, USA and it gets quite hot and bright here... even in partial shade. I’ll research more and report back the progress. Many thanks!

So what did it end up being?

After much research and investigation, I discovered that my 2 boys ages 7 and 8 were "spritzing" the Japanese Maple during the heat of the afternoons. So the water on the leaves was amplifying sun rays to cause the leaf scorch patches and speckles. It may have also been from over watering as apparently, they chose to "give it ice cubes" too in an effort to keep it cool. Their tactics worked for their Junipers but not so much for the Japanese Maple. Thanks for the advice.

The Myth of Hot-Weather Watering

"Watering Plants on a Hot Sunny Day Will Scorch Their Leaves"

https://s3.wp.wsu.edu/uploads/sites/403/2015/03/leaf-scorch.pdf