To get this trophy you need to stretch out boy by holding L3 and R3 in opposite directions and then get 10 inhabitants to sit on him at once. People and animals both count as inhabitants.
There is no 100% guaranteed way to get this trophy as there is no way of forcing inhabitants to sit on BOY, for ages I wasn't able to get more than 5 on him at once, so this is a little luck based. The best advice I can give you is to hold up on L3 and press Start, then select Planet and The Moon. Keep going into BOY's house, hold L3 to the right and press X twice. This will bring you to another map. Keep doing this and eventually you will come to a small map with no vehicles and a lot of inhabitants. Then stretch boy across the entire map and just wait, eventually ten of them will jump on him. If they don't after a few minutes, then just go to a different map and test your luck there. I managed to do it on a map full of Moons and Princes.
As a side note, if a prince jumps on you while trying for this, you will unlock
These maps are also brilliant for unlocking
Please vote positively if this guide was helpful and feel free to leave a comment if I can improve it in any way.