I'm sorry ot hear that your little guy is having this issue. Silky Terriers are a beautiful breed. Hair loss can be caused by many things. One of the most common causes are allergies. Check the label of the food you are currently using. If it contains artificial preservatives or food dyes consider switching to an all natural variety (these can be found at any major pet supply store). Shampoos that are oatmeal based are often gentler on the skin and you may want to try these as well. If you are using a commercial flea treatment, you may want to discontinue that as well. They can also cause hair loss. Finally, take note if there has been any change in his environment. Cleaning products, pesticide sprays, and even landscaping elements (like cedar chips) can contribute to hair loss.
If none of these apply to your little one, you may want to discuss the possibility of a thyroid issue with your vet. Many small breeds are prone to these and fortunately, they are easily managed. I hope this was helpful. If you have any other questions, please let me know. -Robyn