Government Polytechnic, Mumbai

For diploma, they only want your 10th pass marksheet with a good percentage but some of the diploma colleges conducted their own entrance exam.

Yes, with 71%, you can take admission.

75% is good enough to get admission in this course. For details, check link below:
https://www.shiksha.com/engineering/course/diploma-in-mechanical-engineering-government-polytechnic-bandra-east-mumbai-56222

Eligibility:
Candidate must have passed 10th standard from a recognized board.
or
Candidate must have passed 10+2 from a recognized board.
or
Candidate must have passed from I.T.I / NCTVT.
Admission Process
Interested candidates can apply directly to the college.
https://www.shiksha.com/college/government-polytechnic-bandra-east-mumbai-24094/admission

Hi Sir,
You can definitely let him go with diploma. As diploma consists of 3 years and after that, he will direct get admission into the second year of engineering. He will not have to appear for any entrance exam. If he opts for 11th 12th, it will be of 2 years and he will get admission in first years of engineering which is equivalent to 4 years of engineering i.e. 2 years of 11th and 12th, and yrs of engineering(2+4=6) poly is of 3 years i.e. (3+3=6 years). SO, the duration remains the same.
Hope this clarifies your doubt.
Best of luck!