Things such as - if you are stuck on a question, move on. Don’t spend time working out one when you could answer three further on.
Leave the last five minutes and fill in any gaps randomly (if multiple choice) he’ll have a 25% chance of getting it right.
Small things, but make a big difference.
Read widely, know times-tables fluently.
There is always an element of surprise with these things. A child who was sure to get in doesn’t, a child who was borderline does well. Always prepare for all eventualities.
Speak to the pastoral team at the high school, it might seem early days but they’ll be happy to help. Make sure you go to the open days with your DS, and if you still need reassurance book for a tour in school time. You’ll see how the children are, how they move around the school, on a normal day. Most schools IME are very accommodating.
I’ve been through this 4 times. I have learnt to cover all the basis, never be afraid to ask any question, take nothing as a given but most of al, steer clear of conversations with other parents about the 11+. Nothing will get you more worked up than this.
Good luck.