Copenhagen to Oslo

It is not the quickest way, but the train is certainly the most interesting. This www.seat61.com/international-trains/trains-from-Copenhagen.htm#Copenhagen-Oslo has all the information on this connection.

Going across water is not necessary. There is a bridge and tunnel from Copenhagen to Malmö in Sweden called the Øresund Bridge. Details here: https://en.wikipedia.org/wiki/%C3%98resund_Bridge
and yes, for details of the trains see the man in seat 61: http://www.seat61.com/international-trains/trains-from-Copenhagen.htm#Copenhagen-Oslo

Thank you both for your help! I'm going to check on that information right now!!

Note that the train is not only not the quickest. It may not even be the cheapest. Check out http://www.momondo.com/ to see if they can get a cheaper flight.

If they want the ferry experience, also consider http://www.dfdsseaways.com/

Thank you...we are checking on that info also.

SAS hubs at CPH and serves major US cities. If you grandson is near one of the SAS served US cities, he could do a multiple destination booking, fly in to CPH as a stopover destination, then fly CPH to OSL for his second stopover destination, returning from OSL to the States. Note that SAS is day of the week price sensitive.

You may have to use a third party website such as Expedia to do the multiple destination booking. The current SAS website does not appear to have a multi-destination option. I did a similar three leg multiple destination flight earlier this year (SFO to CPH, then CPH to HEL, returning from HEL at $1073 total per person including travel agent fee, but that was for low season January-February travel).