It is definitely reasonable for someone his age and his build to put on a lot of weight really quickly.
When I started college, I was 6'4", 177 lbs. Not much muscle, definitely kind of a string bean. I started lifting way more consistently (even with this, there were three separate times where I took approximately 2 months off for varying reasons), eating a bit more, etc. and less than two years later, I am 212 lbs, which is approximately a 35 lb increase. It is definitely not all muscle, but no doubt a good portion is, and I would still classify myself as pretty skinny and lean. Went from barely being able to bench 135 for 1 set of 5 reps to benching 205 for 5 sets of 5 reps in that time frame. Can't give any kind of equivalent leg measurements because I am still recovering from an ACL tear but I can't even imagine what kind of gains someone with his frame could make given a proper diet and training regimen.