A parent's own sensitivity is an important ingredient in teaching thoughtfulness, and by your question you demonstrate that you have this quality. So you will be teaching concern for others by your own example, which is indisputably the most effective way. But with some children, more than a good example is necessary. You have to make your son aware of the feelings he is likely to generate in others by his thoughtless behavior. Whenever an episode of insensitivity on his part occurs, resist the temptation to scold or preach and, instead, calmly ask, 'How do you think Tommy felt when you said that?' You'll probably get a shrug of the shoulders and 'I don't know.' Follow up with, 'Well, let's think about it. How would you feel if Tommy said that to you?' Continue the questioning without sounding judgmental until he can verbalize something about his feelings.
And certainly, you want to praise him every time he does something that shows respect for someone else's feelings. 'That was really nice of you to let your brother play with your truck; I know you're afraid he might get it banged up.' And don't hesitate to let your own feelings show; he may need to develop better skills of reading other people's feelings. Finally, without making a big deal of it, find opportunities to compliment your younger son whenever he shows empathy and concern for others. Hopefully, in time, your older son will grasp the idea that thoughtfulness and concern for others are important values in your family.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.