To be honest mate, i'm 16 and my body looks like yours does. I have a slight 6 pack, I say slight because from the front you can't really see the 6 pack, but from a slight angle you can see it perfectly fine.
To whoever said he is tensing and you know that how? Because his veins are sticking out. I'm not saying you're wrong as he may be tensing, however my veins stick out alot but that's because I exercise alot and am generally skinny.
Also, people saying he needs to eat more, it may be easy for you to say that, but when you have a fast metabolism and the fat in't going where you want it to, let alone because of the exercise, the fat doesn't really build up, then it is quite hard.