How early can you detect autism?

My son is 20 months old. He has been showing signs of autism for awhile now. I sensed it at 3. Because he’s been through some major trauma with domestic violence and foster care. He is still too young to be diagnosed is what I’m told. The hope is for him to grow out of it once all DCF involvement is gone. What do you guys think, is he too young to be diagnosed with autism?