If you booked that within the last 24 hours and can thus cancel for a refund, I would do so and then rebook on a single ticket IAD-ARN with a connection wherever it best works. As it stands, if your son cannot make it to LHR as scheduled, he will lose is LHR-ARN ticket.

There is absolutely no way to predict what will and will not fly a month out. But, you can reasonably be assured that there will be some connections from WAS to ARN available. If you book and a flight is cancelled or substantially changed by the carrier, your son will either be rebooked to his satisfaction or you may have a refund and you may then book what you want.

The key here is that rebooking and convenience may not go hand-in-hand. I would simply allow an extra day of flex so that, if things get tight, he is not late.