I went for an interview at Oxford about 10 years ago for mechanical engineering, it was 3 interviews instead of two.
One of them was a one on one with a chap discussing a few physics based questions I think, I don't really remember that one.
The second one was with with a lady with a math based focus. I had to essentially prove/explain differentiation from base principles. Luckily we had covered this pretty well in our pure math (I think) modules only a month or two before. There may have been some other math based proofs.
Both of these were quite informal, sitting next to the person on the same side of a desk working on a piece of paper. They were pleasant people and if I got stuck they would help me out to get to the next step. They would ask you about your though process and try to assess how you approached the problem if you didn't know the answer.
The third interview felt quite intimidating, it was in front of 3 older gentleman and I was stood up in front of a whiteboard. They asked me a few questions with regards to interests I had put on my personal statement. They picked out that fact that I was an avid paintballer at the time and created a hypothetical scenario involving a paintball flying through the air and hitting an object. It was the perfect mechanics question relating to transfer of momentum, velocity etc. A lot of the stuff you cover if you do the mechanic math modules. I think I actually did quite well there.
All this was throughout one day, and I was sick as a dog. I felt like absolute dog poop, I had the flu or something. I did my best to be positive and work through it though, without coughing all over them.
They ended up offering me a place on the course if I managed to get AAA at A2 (what I was predicted)
Fast forward to that summer. I got AAB. The B was in Chemistry. They didn't let me in and I was absolutely devastated...
I would agree with the advice above from w11tho.
Tell your son to be himself, be genuine, not afraid to ask questions, show interest and passion about the subject if given the opportunity and to try not to let them interviewers phase him. There's a greater chance that they'll be amicable as opposed to trying to trip him up.
If he doesn't get in, then he has some incredible backup choices.
I sincerely wish your son the best of luck.