If your son has joined a gym and has been given a diet prescription, he can and must stick to it though I don't find any reasons other than restrictions of protein intake that his fish intake has been curbed. You may ask the gym instructor and understand the mechanism in your sons case and the weight loss strategy the instructor plans for your son.
As far as cholesterol in fish is concerned, there may be some cholesterol in some fishes like hilsa but your son can take these on an infrequent basis. He must get into a regular exercise and not discontinue his gym exercises till 1 year after he has attained his ideal body weight, that is 75 kgs.