Hello there,
Your son's getting government polytechnic college and electrical engineering branch depends on the rank which he secured in his polytechnic entrance exam.
Secondly you have not mentioned your Home state.
It would have been better if you have mentioned your Home state.
Because different states have different entrance exam for polytechnic.
For west bengal, the entrance exam for polytechnic is JEXPO.
For UP it is JEECUP.
For AP it is AP POLYCET.
You can go through the link below to know about JEXPO,
https://engineering.careers360.com/articles/jexpo
For details about JEECUP go through the following link,
https://engineering.careers360.com/exams/jeecup
For further analysis about AP POLYCET go through the link below,
https://engineering.careers360.com/articles/ap-polycet

Hope this helps!
Good luck!
Thank you.