You can use a splitter but then the son would have to get his own modem with IP address from the service provider. Other option is to run CAT5 cable from a router in the office to the son's PC, or maybe even wireless, since that stuff is pretty cheap.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma