The table below shows the top 20 in relative SLG for ages 20-21 with at least 800 plate appearances.
The 138 would put him in 5th place and in some very great company. He also has 523 total bases combined in 2012-13. That is already the 16th best total at ages 20-21. I have him on a pace to get 368 this year. Then his total for the two years would be 683, an all-time high. Here is the top 16:
It should not be too hard for him to reach Cepeda. He would just need to SLG about .362. Slugging about .444 would get him to Kaline. He needs to slug about .547 to tie Williams, assuming he plays every game and gets about the same ABs per game he has so far this year.