Anytime I want to register for something online:be it scholarship or any of these online management courses that requires me filling an online document; My cousin's first name, surname and middle names always come up as a default.
Could this be a sign that he's tracking my email ? What could be the cause of this ?

He is not tracking you!

This could happen if he had used your system in the past or still sharing it with you now.

Clear your browser history, cache etc.

Check autofill in your browser and clear all the data there.

If he has a signed in profile on your browsers, sign him out.

He hasn't used my system for once, the more reason why I'm so confused.

Or did you at any time help him to fill out a form on your system?

This is just a matter of your browser remembering what you filled in the past.

Have you tried using another browser?

Whatever it is, try my suggestions in my first reply.

Venice001:
He hasn't used my system for once, the more reason why I'm so confused.

Oga clear ur cache.
This is no how tracking or spying works