How can you write a dialogue between son and mother on the topic of food habits?

Writing dialogue is not as hard as you're letting it seem. You have dialogue all the time -- it's called talking. If you honestly cannot think of what your characters are going to say to one another, you need to go take a break and go somewhere out in public. Sit somewhere in the middle of a crowd for one to two hours and just listen to people talking. Then, go home and write down some of the things you heard people saying. That's dialogue.

When you need to have your characters talk, just pretend it's you and a friend (or several friends), and have them say something you'd probably say in the same situation. Then imagine what your friends would say in reply, and go back and forth that way. As you become a better writer, your characters themselves will "tell" you what they want to say, because they become like real people to you.