You need to deal with a specialist. Call your contact for the IEP. The teacher doesn't know if your son would be able to learn a second language. Only a specialist.
I don't understand why you would need to write his IEP. Both my granddaughter and my grandson have IEP's and they are assigned someone from the county Intermediate Education Office who gathers info and writes the IEP. Then my daughter and anyone who wants further input attends a meeting during which the IEP is discussed and modifications made if deemed necessary.
If you're not working with a specialist in the school district you need to be. Call the school district office and ask for their number.