Praise be to Allaah.
Doing good is a habit and doing evil is a habit. Man is always either making progress or falling behind. When a person gets used to doing something, he becomes very fond of it and loves it, and dislikes anything else. Then he thinks that the thing he is not used to is not possible, but this way of thinking is not right and it goes against both the laws of Islam and the facts of reality. Therefore the one who uses his left hand to eat and drink should be taught the Islamic ruling and should be reminded of the hadeeth of the Prophet (peace and blessings of Allaah be upon him) on this matter. When a man ate with his left hand in his presence, the Prophet (peace and blessings of Allaah be upon him) said, Eat with your right hand. He said, I cannot. He said, May you never be able to! Nothing was stopping him but his stubborn arrogance, and he never raised his right hand to his mouth after that.(Reported by Muslim, 3776). According to a report narrated by al-Daarimi, his right hand never reached his mouth. This man suffered paralysis because of the duaa of the Prophet (peace and blessings of Allaah be upon him).
Al-Nawawi (may Allaah have mercy on him) said: In this hadeeth we have an indication that it is allowed to pray against a person who breaks a shari rule with no excuse. We also see the principle of enjoining what is good and forbidding what is evil in all situations, even with regard to eating. It encourages us to teach proper manners of eating to a person who is eating if he is doing something wrong. The hadeeth also teaches us that the Shaytaan eats with his left hand, because of the hadeeth: When any one of you eats, let him eat with his right hand, and when he drinks, let him drink with his right hand, because the Shaytaan eats with his left hand and drinks with his left hand.(Reported by Muslim, 3764). The Shaytaan eats with his left hand so the person who eats with his left hand resembles the Shaytaan, indeed the Shaytaan is sharing his food with him. Ahmad reported with a hasan isnaad from Aaishah and attributed to the Prophet (peace and blessings of Allaah be upon him): Whoever eats with his left hand, the Shaytaan eats with him. (Tuhfat al-Ahwadhi, commentary on hadeeth 1721).
Moreover, the left hand should be reserved for removing dirt, as when cleaning oneself after going to the toilet (istinja) and blowing ones nose and other kinds of cleaning oneself and removing dirt. How can a person use that which is used for removing dirt and impurities for putting food in his mouth?
Hafsah (may Allaah be pleased with her), the wife of the Prophet (peace and blessings of Allaah be upon him), reported that the Prophet (peace and blessings of Allaah be upon him) used to use his right hand for eating, drinking and getting dressed and his left hand for everything else.(Reported by Abu Dawood, no. 30)
Al-Nawawi (may Allaah have mercy on him) said:
This is a fixed rule in shareeah. It has to do with honourable things such as putting on ones shirt, trousers and shoes, entering the mosque, using siwaak, applying kohl, combing and styling ones hair, shaving the head, giving salaam at the end of prayer, washing oneself in ghusl or wudoo, leaving the toilet, eating, drinking, shaking hands, acknowledging the Black Stone, and other things where we are encouraged to use the right hand or start with the right. As for the opposite [things that are not honourable], such as entering the toilet, leaving the mosque, blowing ones nose, washing oneself after using the toilet, taking off one's shirt, trousers and shoes, and so on, then we should start with the left or use the left hand. All of this has to do with honouring the right hand or right side.
Some people may have a real problem that prevents them from using their right hand, such as being paralyzed and so on. In this case they are excused and there is no blame on them. Al-Nawawi (may Allaah have mercy on him) said: If he has an excuse for not eating and drinking with his right hand, such as sickness or an injury, then this is not makrooh
Most likely the problem of the boy you are asking about has to do with habit, so try to get him used to eating with his right hand and keep on advising him and reminding him to do so. You could seek the help of psychiatrists, because some of them may have some useful ideas. We ask Allaah to give you strength, May Allaah bless our Prophet Muhammad.