Whether your child is struggling with writing because he is young and new at it or its because he is older and had bad experiences that have made him reluctant to write, you can begin at a simple starting point and work slowly, step-by-step until he is writing reports, papers and essays.
It all begins by making a list.
It can be a list of any sort. Have him make lots of them. About anything that interests him. Perhaps you can give him a topic once in awhile. Make lists from the other subjects he is taking such as science or history. Then it can count for two subjects and make your student very happy. Slowly begin to ask him to add in some organization to his list making. He could list his ten favorite cars in order from the best down to his tenth favorite. He could list the colors in a rainbow in order and then list a flower that is of each of the colors. He could list how he plays a particular game in likes to play in the order it is played (what do you do first, and so on). Have the ordering be in as many different ways as you can think of. Help him through any that are difficult for him, by asking questions to guide him.
Once you think he is ready, have him pick one of the lists to use for the next type of writing project, the paragraph, but first let's look at sentence writing.