I'm an entry level employee at a defense company and on a project I was working on I was tasked with adding features to a kernel module given to us by a third party. Prior to working on this project I had not worked with Linux or kernel modules and I was very lost. One employee working on the project, who knew a lot about what I was trying to do, went out of his way to help me. He is experienced and has worked for the company for 10 years. He works at different site from me, several miles away, and came out to help me several times. Every time he did he would spend hours with me explaining things and would help me write code.
I think he did a great thing in helping me when he also could have spent his time doing other things. I want to show my gratitude somehow and I think he deserves more than a simple "thank you". What is an appropriate way to show thanks in a professional environment?