Replacing a boy's head with a watch may indicate that he values time or pursues a regular and planned life. Watches often symbolize time and discipline, so it may be that boys want to express their attention to time management or pursue an orderly arrangement of life by changing their avatars into watches.
This may also be the image he wants to convey to his friends and followers. He pays attention to the concepts of time and efficiency. But the specific meaning should be interpreted according to the specific situation and personal habits. If the specific intention is not clear, it is best to communicate directly with the boys and get an accurate explanation.