It is definitely possible then. The cap estimate doesn't take into account sublevels, so if he has any sublevels in passing, handling and OD (which obviously he does), then that 17.7 could easily become very close to 18.0
You might be able to still get him that OD pop, but it may take a long time to train.
It's unlucky that he probably has such a low potential level within the star range. :(