I'll let you start it on a bottle.

"All right," he said. "You go back to the house and I will bring the
runt when I come in. I'll let you start it on a bottle, like a baby.
Then you'll see what trouble a pig can be."

Although I have a general meaning which is probably sth like " I'll give you a bottle of milk and you will do the job". The form is hard to understand for me.

Could please you explain for me if:

"start it" is a some sort of set expression?

"on a bottle" - why "on" ?