Call them an "UNDERGRAD". This means they have graduated high school, and are now studying to earn their "Bachelor's Degree". Unless your brother has actually already earned a bachelor's degree, and has continued at college to pursue a "Master's Degree", in which case you would call them a "GRAD STUDENT". A grad student who wants even more education than that is going to become a "DOCTORAL CANDIDATE", and will earn the title "DOCTOR OF (whatever the field of study is)", earn a Phd certificate, and will probably become a professor who teaches at a university.
So, you would probably say: "My brother is an undergrad in Boston.", and people will immediately presume you mean Harvard, and be impressed, although they should not be. Lots of bad people also graduate from Harvard....
Once he graduatues, you'd say:"My brother did his undergrad in Boston." This would be a very colloquial, very native-born-speaker-of-Americañol style of speaking.