The 2017 Marucci Hex Connect -8 Senior League Baseball Bat: MSBHCX8 in a 30 inch model would be best for your player. This bat is a two-piece fully composite design with a balanced swing weight and would be an excellent bat for a smaller 13 year old. Would certainly recommend a -8 model for a 13 year old as this will help him transition to BBCOR bats that are required in high school (-3 length to weight ratio).
Marucci will not be releasing a 2018 version of this 2017 Marucci Hex Connect -8 Senior League Baseball Bat (MSBHCX8). The next Hex Connect model that will be released is going to be a part of the 2019 release. That release should come out in the summer/fall of 2018.
Unfortunately, the 2017 Marucci Hex Connect -8 Senior League Baseball Bat: MSBHCX8 is only available in a 2 3/4 barrel. However, the 2017 Marucci Hex Connect -5 Senior League Baseball Bat: MSBHCY5 will feature a 2 5/8 inch barrel, but it will be 3 ounces heavier.