The treatment for selective mutism is a multi-modal approach - usually involving intervention, therapy and possibly medication. If the social anxiety/selective mutism is quite severe, medication will be used to "take the edge off" so that the other methods can be affective. Other therapies as art and play might also be of use - in our area, our psychologist will not attempt CBT with a child until 8 or 9 years of age. So, it is not the medication that will help your child learn to "manage his fears" but the therapy and intervention methods - the medication only lessens a child's anxieties so that these methods can work. Then, when a child can control and manage his fears, the child is weaned from the medication. Hope this helps ..
Prozac can be very useful. However, it may not be necessary to employ medication. The first attempt should involve cognitive-behavioral treatment, and the clinician will likely be versed in this. If the use of CBT does not prove sufficient to remedy the difficulty, pharmacological treatment can be employed as an additonal component of the treatment plan.