Dear Sir- I would use his name- Dear Mr Xxx
but it was pretty different what I thought to be- it was very different from what I had been expecting
Things changed dramatically due to the merger and loosing a contract with xyz where it made me stand on the rope to make decision. Wish you were there to teach me or Mr. x. Anyway, life has to move how it is assigned. Just pray for me to achieve my goals in the future as I am interested to learn something new always. This part is very unclear to me- why are you writing this? What is the aim of the letter?