What should a man do regarding his son, who has autism, lives with his non-Muslim ex-wife in a non-Muslim country, and he wishes to return permanently to his home country, knowing that his ex-wife refuses to allow him to take the child, and his family is unaware of the existence of this child?

The father must spend on his son, care for him, raise him in Islam, and protect him from disbelief. You have fallen short in this by choosing your wife, and then by neglecting your son's upbringing and leaving him to his mother to raise him as a Christian. You must repent for your shortcomings and try to rectify what has passed by asking about your son, making him love Islam, enrolling him in an Islamic school, strengthening your bond with him, and trying by any means to take him with you to save him from disbelief. The fact that you are married and your family does not know about this son is not an excuse to neglect him. Therefore, fear Allah and do what you can to save him, even if you have to delay your travel. If you do travel, return to him from time to time and do not lose hope in his conversion to Islam and his righteousness.