There are several caveats (Aviation, Cyber, Medical School, etc.), but the simple answer is that he will attend an officer basic course for his branch after graduation from USMA before reporting to the assignment selected during branch night. That course will vary in length by branch, but will be about 4 - 5 months, and completion will make him branch qualified and able to assume duties as a 2nd Lieutenant in that branch, usually as Platoon Leader. All branch courses will have some common material with the differences being material specific to the assigned branch.
Anyone accepting an appointment to West Point should be prepared to serve in Infantry, Armor, and Field Artillery. There are minimum percentage requirements for each class to fill combat arms slots and those three have the largest allocations. Also, many who choose and get non-combat arms branches such as Finance, Military Intelligence, Quartermaster, etc. are "detailed" to serve their first tour in a combat arms branch. Although it can vary greatly by year and branch, you should expect about half of each non-combat arms branch to be detailed into a combat arms branch.
Aviation and Cyber have much longer courses and follow-on assignments are made during those courses rather than during branch night at West Point. As Charlie mentioned, a new officer may also attend other military schools - especially Ranger school for Infantry officers. West Point grads usually get priority on BOLC dates and normally get to their first assignment 6 - 12 months after graduation for most branches - Aviation and Cyber longer.