Whether or not he attends SLS will not matter to his application. Plenty of people who did not get to go to SLS receive appointment.
All true, from a purely WCS perspective. If it's truly a choice between Boy's state and SLS, only the former will help your WCS.
But SLS is worth trying to go to if at all possible for several reasons:
- It's the applicant's best opportunity to get the "vibe" of USMA and be exposed to what will become his potential peers and cadre. Remember, USMA does not offer overnights unless appointed, unlike USNA
- It's a great chance to get an unquestioned CFA
- It's a great opportunity to get a positive input. Which by itself will not get you in. But can counter a concern or unanswered question about the applicant intangibles.
Though it does not help with WCS, and many get in without, it's worth saying that of the people who attend SLS, a very large percentage receive appointments.
That's probably more due to top-grading the participants. IE: They are often are by definition the most competitive in order to be selected. (the exception to that are those who did just OK on PSAT, but rocked the SAT/ACT). The figure mentioned in briefings at SLS seems to bounce around 50% or more of attendees are ultimately offered appointments. It may vary by class of course.
SLS is not magic, you may not find all the workshops illuminating. USCGA AIM is probably more fun. But given you are about to commit 9 years of your young life.... it's really worth trying to attend if offered.