I will suggest your son to start getting familiar with Indian laws as it might be a long battle for him, also because he has long life in front of him and he is emotionally attached to his son. There are some factors you need to consider
1) To fight legal battle in India, you need to be in India (either frequently or for in a stretch) which will make you spend money
2) Your current status of NRI makes you prone to fraud/ cheating/ misleading in the hands of lawyer
Regarding Recovery of loan
1) There are legal provision (please note that it is no guarantee that you will get your claim) where you can initiate processes. Refer one sample judgement here – http://delhicourts.nic.in/Nov08/DEEPA%20JAIN%20VS.%20RAMA%20KANT.pdf
2) Both you and your son can initiate appropriate process. But please consider above factors before proceeding further. It is no guarantee, particularly in the absence of any written agreement.
Regarding Child maintenance demand/ child custody
1) Agreeing to such a demand of ex-wife is against the Indian way of life. Better wait for her to initiate a maintenance claim suit through court and proceed further unless you are compelled to pay because of your own emotional attachment.
2) Your son can claim for child custody subjected to the above consideration of your limitation of physical presence and prone to being misled by lawyers and end up spending few thousand dollars.
3) Something that can be in your favor for claiming child custody is (if ex-spouse is living with her parents) her alcohalic father and her spending time with her boy friend, again subjected to enough evidence.
If you have any relatives in India in the same city where ex-spouse is staying then it might help you a bit from logistic perspective.