The biggest difference between the two bats is how they are constructed. The 2018 Easton Ghost X -8 USA Baseball Bat: YBB18GX8 features a two-piece, fully composite design while the 2018 Easton Beast X -8 USA Baseball Bat: YBB18BX8 features a one-piece, alloy design. It is hard to say that one bat performs greater than the other because every player is different. However, I will say that I have seen higher product reviews on the 2018 Easton Beast X -8 USA Baseball Bat: YBB18BX8.
Based on your sons height and weight I would recommend taking a look at the 2018 Easton Ghost X -10 Senior League Baseball Bat: SL18GX10 in the 30"/20 ounce size.
AABC, Babe Ruth / Cal Ripken, Dixie, Little League, and PONY will all be affected by the USABat rule change, but USSSA will not. Currently, USSSA Baseball will allow the use of a bat that carries the new USABat stamp (as long as barrel diameter, drop weight, material specifications, etc. match the specific division's requirements). The 2018 Easton Ghost X -8 USA Baseball Bat: YBB18GX8 might be too short and light for your player. Some of the leagues allow the use of a BBCOR bat which features a -3 drop weight. I would recommend talking with a league/tournament official to see if he can swing a BBCOR bat because that will be of more benefit to your player. The 2018 Easton Ghost X BBCOR Baseball Bat: BB18GX would be a good option if you are approved.