While both bats will feature a one-piece, aluminum design and an end-loaded swing weight, the barrel diameter and league certification will be different for these bats.
The 2018 Easton Beast X -5 Senior League Baseball Bat (SL18BX5) has a 2 3/4" barrel diameter and will be USSSA-certified (1.15 BPF). The 2018 Easton Beast X -5 USA Baseball Bat (YBB18BX5) has a 2 5/8" barrel diameter and is approved for USA play.
USA-certified bats are lower performing than USSSA-certified bats.
I would say that your player is probably too strong for the -10 and -8 bats. I would recommend this 2018 Easton Beast X -5 Senior League Baseball Bat (SL18BX5) in the largest 32"/27 oz size. I would believe that he should be able to get solid durability using that bat. However, if the -5's get to be too light for him, the next move would be to the -3 BBCOR bats used in high school play.
The 2018 Easton Beast X -5 Senior League Baseball Bat: SL18BX5 is a one piece alloy bat so there is potential to dent this, much like a composite bat has the potential to crack. However, the Beast X utilizes ATAC Alloy to create the strongest and most powerful barrel Easton has ever released.