1) your brother is 5th class hence you can make a model of phases of moon like crescent moon,half moon etc
2) you can make a model of lunareclipse or solar eclipse using a torch as sun and a medium sized blue ball,coloured with land on it as earth and a small white ball as moon.by using these materials you can follow the procedure of lunar or solar eclipse
3) you can show a model of air has weight with a scale and 2 blown balls and a thread.
you need to tie a thread in middle of scale and hang to blown balloons each end.now take off air in one balloon and we see that the balloon which has air goes down because it has more weight than the balloon which has no air.
best of luck with your model!!