Our daughter was accepted to Chemical Engineering at UW, and when we toured there they reiterated that if you’re not accepted directly into engineering it’s virtually impossible to switch into engineering. She’s also been accepted to UCSD for Pharmacological Chemistry which is honestly a better fit for her.
UCSD is big enough that I think he could find his own groove without following after his sister, if that’s his concern (my daughter’s BFF has a twin brother who is not pleased she’s considering the college he is planning on, but there are fewer than 2000 students there, so they probably couldn’t avoid each other).