"It's a shame I won't be able to say goodbye today. Could you please pass my farewell onto him? I want to thank him again for all the help he gave me while I was a newbie. Every time I asked, he came to help me out. I will send him an email later too. Regards, xxx."

Hello Vivian, This is very nicely written. "Pass my farewell" is one of those beautifully elegant, slightly old-fashioned phrases. I like it. There is only one mistake that stands out to me, which is in "I want to say thank you again for all his little help for me while I was a newbie" You can't use "little help" in this way. I can see what you're trying to do, and it's possible in many other languages, but not in English sadly. I can think of two options: 1. Simply take out "little". 2. I want to say thank you again for all the times he offered me a hand while I was a newbie. "A hand" contains the sense of "little act of help". Otherwise I'm not quite sure about "grabbed" as it's slightly unclear (can you grab someone who isn't already with you? In which case he wouldn't need to "come").