Is it a good time and age to take IQ/aptitude test of my son to know his interests? He is in class 8th right now.

Probably, he is too young to go through an aptitude test. Give him some time and let him pass through this moments of his life. if you are willing to give him an aptitude test wait for another 1 year and then you can go ahead. If you want more of his interests to be developed, and then choose a care