Given the very close proximity here (your ex's protestations of the burdensome distance notwithstanding), I don't see why your son can't do what he wants to do with his dad (tennis, walks, just hang out, whatever) and still keep your house as "home base." Are there busy highways or something between your homes? Because it seems like 3.5 miles would be a very short distance for a 17 y/o to walk or bike.