When should we start preparation for IIT?

Your son is just 5thclass.HEcan try solving problems on basic formulae &topics.Later,he can try twisted problems.Now its not necessary to force your child

It is really good to take up a course of that sort. But make sure the child is interested it in and has the potential to do the course. First find out what is his ambition and if he wants to do it. Try not to imply your decisions on the child or force him to take it as it'll not help the child.