If the kid wants to be like Hulk, so be it. Just don't let it go too far, or in other words, don't let the training afect the kid's growth.
It can't be bad if the training is in healthy proportions, look at Shaolin Monks, they begin their training at very early ages and their training is very hard (has I seen on documentaries).
how many shaolin monks have abnormal muscle mass at that age? none cause as hard as there training is its to train the body in discipline and toughness not in body building
they hang themselves at the age of 12 to train there neck strength and at the age of 5 they begin flexibility training which is actually good for growing since it stop development in bone disorders plus they are taught in the art of shaolin judo and kung fu.
this kid doesnt smile at the camera when he poses and lets face it a kid is always smiling when getting attention, his dad is clearly obscuring the boys vision in terms of him thinking he can actually become the hulk this way