My dd at her 11+ exams was asked things like what does she do as a hobby and why does she like it, if she's in any clubs/teams at school, they asked her about the badges on her school jumper (school council, hall monitor badges, she described her roles),
maybe talk about a recent piece of work eg. a poem written at school or something, one place asked her what she was doing at the weekend, she said she was going to the airport straight from the interview to see her grandparents in Spain for a few days for a "much needed break" (ha!) - not a lie, she was, that got a few laughs from the interviewers.
One school got her to do a few Maths game type things because her maths was a weak spot in the examination she had taken for the school and although she admittedly didnt have a clue what to do (she'd never seen/used that particular type of "Game" before) she just tried her best but explained she hadn't done that maths game before and but that it "seemed fun" (haha). It was really a very very selective, academic school and she got a place through Reserve list so she must have come off OK despite floudnering over it - so if they are polite, chatty etc I think it goes a long way.
just tell him to be himself, if he's really into Xbox, then don't lie and pretend he reads Shakespeare after school for fun, say why he likes Xbox, try and expand a little bit on his answers - I like playing Rugby BECAUSE I like being part of a team AND I enjoy being with all my friends - that's what we worked on with dd, expanding a little, just give him a few examples and then get him to come up with his own.
Most of all, make it fun, make sure he's dressed nice and smart so he feels smart, say you'll go to McDonalds/A museum/let him have a friend round for tea that night/whatever's a treat for him, and it's a little incentive.
Make sure YOU stay calm, are relaxed and make it seem like a breeze - pressure does NOT help 10 year old boys!!!
Good luck!!!