Your son must, must, must read the instructions carefully. All of them.
It is very likely that there will be marks for working in maths. If so he MUST show his working. A correct answer without working will not get full marks. This can be hard for some boys as they can just "see" the answer.
Revise data, probability and geometry as these usually are tested in 2nd round. The rest are likely to be wordy multiple stage problems. Historically Sutton sets a harder maths paper than Wilsons.
It is helpful if your son practises 30 and 50 minute writing exercises in a variety of styles, eg diary, letter, recount, description, essay. Teaching him to plan will help. We sat down and made sure that ds understood what literary tools he had at his disposal, so that he would look to use a variety of sentence structures, language, punctuation etc.