@Era congratulations to your son. Well, Durham is small, great as you can everywhere you need to and it has pretty much everything you need. The colleges break down into new builds right at the top of the hill with South and John Snow (my child was in South) so modern, most en-suite or a shared bathroom between 2 rooms. Opposite are Josephine Butler and Stephenson which are behind the park and ride (you MUST get on the bus if you use this, even if you buy a ticket, there is no park and stride, I have no idea why).
https://www.durham.gov.uk/article/22364/Howlands-Farm-Park-and-Ride
If this was me going up for the first time I would park on the park and ride, get the bus into the town centre, have a mooch round there, lovely, quaint, see the cathedral and the castle at least from the outside. Walk down to the bailey colleges and then walk down to the science site where lots of buildings and departments are. There is another site a little further down too. As you walk back toward park and ride all the other colleges are off that road (South Road) with your final 4 colleges at the bottom. Have a walk round the college sites too.
There is a massive sports place called The Graham Sports Centre which is to the right of all the hill colleges if you look on google maps. It might be important to your son if he is sporty or likes watching it.
Please remember that Durham will feel very quiet as the students are not there. It has a much more vibrant feel when the students are all back. Ds found random students willing to help carry your shopping walking back to college and have a chat and he also did the same for others. Ds is not sporty nor a party boy but has enjoyed his time. He is going into his final year in October. Any questions, feel free to ask whatever they are. Rooms tours for the colleges etc are on Youtube.