My son is pretty skinny. He doesn't have the stocky build that I got. Guess he'd better start eating more ice cream. He's got a shoulder injury that bothers him a lot too,
Don't know much about seals but am familiar with Special Forces.
I doubt he will make it with that shoulder injury. Before he signs a lot of years away he needs to insure that his shoulder is 100%. This is just a sad fact that he needs to consider. He can totally destroy his shoulder for the rest of his life by pushing too far when it is time to let the dream go. I told my son over and over again to go medical and recycle if injured and he got one foot stress fractured two days from the end and made it on ibuprofen the last way (SF selection). Foot has healed fine.
I was skinny. 6'3" and 158 #. The lack of upper body weight makes it easy to do the pull ups!
Plus being as most of it is leg endurance I did not have a lot of weight to carry. A lot of the bulky guys will stress fracture their feet early in the weeding out process.
As was mentioned, it is about mental will power.