If you have previously recommended this thread, you should see a tick / check mark on the recommend button. Click the tick to undo the recommendation (the tick may appear to change to a cross as you do this.) If you added a comment with your recommendation, you will need to delete that from your facebook wall separately.
Lucky you OP I don't think you need to worry as boys don't tend to flash their bits about to each other once they are fully grown! I have 2 sons and a stepson and I don't think they have ever seen each others willies! Also, I may be wrong but the size now might not be in proportion to their size when they are fully grown so your DS might catch up when he is older.