Renewing passports is really easy, just do it to your sons address a few months before they run out so you have time to pick the new ones up. If you are careful to request paper bank statements and anything else you may need when you know you'll visit your son, you will be fine. Register at a dentist and doc near him and time appointments. Otherwise use nhs walk in centres or temp patient where possible. My gp has even done prescriptions over the phone and faxed them to the nearest chemist.
I don't live aboard but I'm a property guardian and move a lot, no fixed address, utility bills, council tax etc. I've been fine like this for years, the only thing that was a pain was changing banks but if you can be on the electoral roll at yours sons address it will help a lot.
Would drawing up a rental agreement for a nominal amount (or just don't pay) between you and your son help to separate your credit at all?