AFAIK, there is no Android hot-spot public API. However there are some projects using reflection (java.lang.reflect) that allows to control Hotspot, maybe you could try this one https://github.com/nickrussler/Android-Wifi-Hotspot-Manager-Class and develop your own startup app.
You can also try to simulate user input, but this is a flawed solution.
$ am start -n com.android.settings/.TetherSettings
$ input keyevent 20 & input keyevent 66