where is his head in relation to the top of the seat? although weight limits on car seats are absolutes, height limits for car seats are more of an estimate because some kids have longer torsos and other kids have longer legs. A rear-facing seat is outgrown if there is less than 1" of hard plastic shell above the child's head (this link gives a good illustration of how to measure that 1": http://www.car-seat.org/showthread.php?t=30201 )
Also, please note that most child passenger safety technicians would advise against buying a secondhand child safety seat unless it was from someone you knew personally and trusted to have used and taken care of the seat correctly. If the seat has been in an accident or the harness straps have been submerged in water or machine-washed or dried, the seat's ability to function properly in a crash would be impacted.
Good luck!