I would probably stay with a 28"/18 oz model. The 2 oz difference should help a fair bit, however only you would have the best guess since we have never seen him actually swing. A 27"/15 oz seems rather small for a player his age however, and would not be recommended unless he was about 4'2" or shorter.
Yes, if you purchase the used 2018 Easton Ghost X Hyperlite -12 Senior League Baseball Bat: SL18GXHL12 you can use the bat during the 14 day guarantee and if you decide you don't like it for any reason, you can return it for a full refund or exchange.
With composite bats, like this 2018 Easton Ghost X Hyperlite -12 Senior League Baseball Bat (SL18GXHL12), we always recommend a break-in period.
On the first day of use, I would suggest taking roughly 100-150 swings off a tee, rotating the barrel a 1/4" every 15-20 swings. On the second day of use, I would recommend hitting soft tosses again, rotating the barrel every few swings. After doing this, the composite bat should be broken in!