The reason why I am writing down (The phrasal verb "write down" is used with a direct object, as in "I wrote down his number") about him is that he is leaving soon. He is moving on to bigger and better things next month. Absolutely, he does deserve it. However I’m a bit of gloom (maybe "I'm a bit gloomy"). The man who is going to take his position is not bad to work with, but my boss is so nice. No one can be replaced him. I will never work for such a capable boss in the company again. I’m not sure someday whether I could be a leader, but if I could become one (the verb "become" should be used with a subject complement, an adjective, noun, or pronoun like "one"), I would want to be like him. To congratulate him for his leaving to a better place, tomorrow at dinner his leaving party  ("going-away party" might sound more natural) will be held. I’ll say that (We will not use "that" before a direct quotation) “Thank you” and “Looking forward to working with you again.