At this age, he needs a changeup. My son is a lanky, featherweight 12 y/o lefty that doesn’t throw hard but is quite accurate. The first two grips he learned were four-seam fastball and the circle changeup.
For a while, I couldn’t tell the difference in velocity between the two, but everyone always said, he has a nice changeup, to which I would often gripe to myself in my head, yeah, but you can time his throws with a sun dial. Now that circle changeup is visibly slower, and he locate it pretty well. It can get kinda filthy.
I guess the point I’m making is that at 9-10 work on a FB and a changeup. Let him see the havok a well timed changeup can induce. My son smiled from ear-to-ear everytime someone lunged and looked stupid not being able to stay back on his.
You can then gradually introduce more grips. Since our pitching journey began he has integrated 2-Seam, Cutter, Slider, and Beginner Curve grips. He hasn’t really used the last two in games, but he is working on locating the Slider this summer for that special batter you need to get out but can’t over-power.
Edited to add:
Perhaps the reason he’s encountering a lot of two-strike hits is that he isn’t varying his effort and therefore his speed. Pitching is an art too. While it is commendable he’s throwing his hardest all the time, I know my son didn’t, and probably still doesn’t, a pitcher has to know when to take a bit off or put a bit more on when it looks like the hitter is zeroing in on his timing.