Bless you for taking him in. Yes - THERE IS A REASON FOR THIS....
I have to admit after reading your story, I don't think you will find his 'owners' - so he may be meant to be apart of your family, which is probably going to be a good thing
I think you need to get him to the Vet - for shots and neutering. This really is the BEST thing for him.
Yes neutering will help with the Marking. But in the meantime - BELLY BANDS and MAXI PADS - This will catch any pee from marking, and Max won't be able to follow b/c he won't be smelling the pee that the new one tried to mark. (Belly bands can be only a temp solution until he is neutered and he gets used to your house and potty routine, etc....)
But with the use of belly bands the new one will soon realize that his "marking" attempts are NOT WORKING - and slowly they will stop - along with the neutering.
I wish you the Best of Luck - He sure is lucky to have found you