If i were you I will just leave since there is so much concerns and uncertainty. Most important you must be happy at a place where you spend a big part of your time. Of course, I will secure a job first before leaving.

In the first place you shouldn't have told him so much, just give him the letter and tell him that you have found a better opportunity and thank him for his guidance during the period you are in the company.

For now just accept whatever he has offered you, and continue to find a job. Also, please do not be so outright in explaining to the interviewers why you are leaving if not you will not be able to find a new job.