To make a more informed placement decision after he chooses a college to attend, he can try the old final exams of the courses he is allowed to skip with transfer credit, so that he can check his knowledge based on the college’s standards.
Placing ahead if he knows the material well will give him more freedom to start upper level course work early and take more free electives (whether additional advanced courses in his major, or anything else in any other subject that interests him). Having more advanced course work earlier may give some knowledge advantage when seeking major-related jobs and internships for summers. It may also give the option of graduating earlier, or completing a coterminal master’s degree without needing additional semesters if offered at the college. Or the advanced placement and credit could just let him avoid needing more than eight semesters if he wants to take lighter course loads than the sample eight semester schedule for his major lists (in many cases, the sample schedules for engineering majors show more than the usual 15-16 credits per semester).