I don't know what you have or have not already submitted so hopefully you have already provided your relevant Pay Stubs and T4's from that job and if not, collect them for submission or re-submission. The purpose of the Employment Letter is to have your employer confirm that you were employed with them (at least up to and on the day you applied) and that your position and job duties qualified under the NOC job classification you applied under.
I would do the following:
1. Keep a screen print of your request for the letter and his negative response. It is proof of your first denied request from that employer.
2. If this person was just your manager and there are others in the organization you can go to, like Human Resources or PayRoll or his boss, request the Letter again from him or the others. Include a polite explanation of what he/she or HR/Payroll needs to include in the letter and that it is very important for your PR and IRCC is specifically requesting the info. (You could include a copy or portion of IRCC's request to you.) Provide a date by which you need the letter. If he/HR again says no, keep a copy of the email reply to submit with your other documents. It will show that you have made more than one attempt to get that letter from your previous employer. Include the contact info (phone, address, email, name, title) for that employer, should IRCC wish to follow up with them directly (I really don't know if they will, but it can't hurt).
3. If you have any of the following, gather them to include with your Letter of Explanation stating that although you have made several attempts for the employment letter (you can also refer to and/or provide proof of earlier attempts you made to get an Employment Letter back when you were originally applying for TR2PR), your previous employer has refused to provide it, and you offer the following attachments as the best supporting evidence you can provide without his/their co-operation (in addition to the earlier Pay Stubs & T4's provided with you application).
- ROE's from that employer (Record of Employment). They are legally required to submit one to the government within 5 days of you leaving the company and if they did not provide you with one, you should be able to request it for free at a Service Canada office with valid ID and SIN number. They should print it for you right at the Service Canada location. (If you already have submitted Pay Stubs & T4's, the ROE may not be required.)
- Do you have a copy of the original job offer letter or email stating your job position/title and at least some duties? Or a job description detailing your duties in that role? Perhaps you have a copy of the hiring ad or job description from when they posted a hiring ad for your job? Perhaps they have a current hiring ad or job description posted on their website for the same position/title you used to have? Do you know anyone who still works there who could get you a job description? In the absence of any of these, do you have any letter or email from them where they even slightly mention your title/role/job duties? Maybe you have some emails where you are given some instructions relevant to your job duties? Does the company have a website or webpage describing what they and their employees do? Depending on what you find there, perhaps it would at least generally support your role.
Submit what you can collect in an organized way with a Letter of Explanation that your previous employer was uncooperative and that you have made your best effort to provide IRCC with what they need to process your application. Make it easy for them to understand what you are submitting. Best wishes.
(I am not an expert in this area so you may also wish to discuss this with a lawyer/consultant who has dealt with this but these are the best substitutes I can think of without having the ability to legally compel the employer to provide an employment letter. You will have shown your 'best effort' to provide IRCC with what they are asking for.)