It is nice that you are considering investing for your son’s education now. The cost of a good four year engineering degree is about 4–5 lakhs per year. This comes to Rs 20 lakhs in today’s value.
A two year post graduation could cost anywhere between 10–15 lakhs.
Add these two numbers and adjust them for inflation. That’s the amount you would need. It could range from Rs 50 lakhs if it is 5 years away to 80 lakhs if it is 8–10 years away. We are assuming education inflation to be around 10%-12% here.
So it boils down to a monthly SIP of about Rs 50,000 to Rs 60,000 depending on what you exact numbers are!