all you need to do is find somewhere quietish to park, he can crouch on the floor of the back seat to go while you and dh stand at either window backs turned. take plenty of soapy paper towels for washing hands afterwards as soap is better then antibac gel for getting rid of germs.
On the A1/A1M !!!!! Really, you don't normally get a lot of warning, when you have to go , you go !
Most of the A1 till after Newcastle doesn't even have Hardshoulder! Then there is approx 20 miles of 'average speed' roadworks with no stopping at all. Then the traffic gets really heavy as you get further south.
A68 -Slower, loads of places to stop, but not good if you feel sick, get stuck behind a caravan or lorry and it can be miles before you can pass. Then you still join the A1.
M6 a bit longer journey , Hardshoulder , can be very busy.
The roads will be busy as every one travels home after the bank holiday, it could easily take 8-9 hours . I really hope your son feels a lot better in the morning, for everyone's sake.