Often some inherent diseases can be cured if noticed early,like inherent amblyopia ,when a baby was three monthes can be noticed ,and if see a doctor ,the problem can be solved.

But my son is already 5 years old ,does too late to take actions? And can amblyopia be cured in a safe and painless way ?

Better late than never, I think you should take your child to the eye doctor for advice, In my opinion, 5 years old is not too late for kids to correct amblyopia, but you should come to the doctor right now, kids eyes need to be taken good care of! Good luck!