At the same time, encourage him to use this disappointment as motivation to work for better grades as that will keep more future opportunities open than allowing his grades to slide.
Knew many HS classmates who were quite disappointed in their choices and after spending a semester confirmed their suspicions their local public colleges weren’t a good academic fit for them.
However, they used their disappointment to work harder to earn top grades, join ECs, volunteer, and find outside scholarships so when they decided to go through with transferring, they laid the groundwork for their transfer to more respectable/elite colleges(including some Ivies) as incoming sophomores/juniors.
Another thing to keep in mind was they all managed to do it without involving their parents beyond asking for signatures for FAFSAs or other forms.