There are many ways to express this, but to focus on the ways that use the words and expressions in your example:
"To a large extent" is fine. It's a standard idiom that means "mostly" and works in this context.
However, "to his credit" seems a bit off. I credit someone with something positive, for example:
I credit him with helping me invent the rotary engine.
But "to his credit" describes a positive attribute or action of that person, and not necessarily what he has accomplished for someone else. It's also often used as an excuse for some other, less positive attribute:
He's not a good speaker, but to his credit, he always speaks the truth.
So in this context I would say something like:
I would like to thank my supervisor, Mr. Rogers. To a large extent I credit him with helping me find my way into the field of theoretical physics.
"I am very grateful for it" is fine but awkward. You can say instead, "for which I am very grateful". The reason I say awkward is because it's not clear what the pronoun "it" refers to. If you like you can state it explicitly:
I am very grateful for his help.
Other ways to express the same sentiment:
I would like to thank Bob Marley. For the most part, it was due to his encouragement that I got into the music industry and I owe him a debt of gratitude.
I would like to thank Arnold Schwarzenegger. It was largely because of his example that I chose to become a professional bodybuilder, and for that I am very grateful.
I would like to thank the Queen of England, without whose inspiration and advocacy I would never have been able to become monarch of my own country. For this I owe her eternal thanks.