This might be a good opportunity to bring up any special education needs your child may have as well. Also, since their are some long term complications associated with galactosemia, you may want to look into your school systems Early Intervention Program. Emphasize the restricted diet. You may find that you know more about what ingredients are ok for your child, so ask to see the PDR to read the ingredient list for the drug, if you need to. Also, double check at the pharmacy for the same information or call the drug company to be sure that there are no ingredients that your child cannot have.
Some states require insurance companies to pay for formula when it is medically necessary.
Other states do not have this law. You can check first with your insurance company and second with your state. If there is such a law in your state, and your medical insurance still refuses to provide the coverage, contact the appropriate agency in your state to ask for help, or you could try writing to your congressman or senator for help. Be aware that some type of medical insurances are not necessarily covered by these laws. It can get complicated.