Bernadette. Though you use it as a surname now, it's clear from these docs that Ontong was his first name. Probably his only name if he were a Javanese. The Van Batavia just means that he came from Batavia or Jakarta as it is now called. The Dutch gave him that as a surname, (which appears first on the docs) but he must have never really accepted it if he reverted to Ontong.
You are very lucky to have gotten as far as you have, but I have to really upset you and say that it is very unlikely that you will ever get any further. If he had only one name, then it will be impossible to trace him, unless you had HIS parents' names, and even then, the 'inlanders' were of no consequence to the Dutch. They made no attempt to record their existence. The Van Batavia probably means that is where he was acquired, too. He could have come from outside the city, or even from further afield. It is an amazing story, though, that he was able to buy his freedom. Good on him. He must have been a strong and resourceful man. Glad to know that he has not been forgotten. His story deserves to be told, on behalf of the many others who suffered similar fates.