He gets the tough treatment. Including from some instructors.
From a pedagogical and sports medical point view I think there is room for debate. Although I am personally a great advocate of Japanese style jûdô, I have some concerns about both the psychological well-being and the medical risks. When one is thàt overweight there are metabolic issues. Most Western specialist would advocate a change in lifestyle, nutrition, and regular aerobic exercise to bring him back to a more acceptable body mass.
There is a huge risk for injury, certainly if adults are going to use their mature bodies and much higher force to compensate for his mere weight issues. He remains a kid. The fact that he is sometimes crying or in pain to me is not at all because he is a weakling. To drag such a mass around and even through minimal exercise will require a huge effort of his still immature body. It does seem that participating in competition contributes to a positive psychological development in terms of self-confidence, but that is generally the case in any sport as long as ... he is the one winning. But it also could turn out the other way and be one loss after another unable to win even a single fight in which case the overall effect would be even worse. But let's open the floor for people's opinions and views.