Version-control histories, change-request approvals, internal documentation with his name on it and training sessions he delivered. These show authorship and duration better than a manager's opinion letter.
Let the audit trail speak
Hypothetical example: a Quispamsis firm's model lives in a spreadsheet-and-script environment maintained over ten years. The petition should include exported version histories showing the analyst's changes by date, change-request forms he authored, model-validation reports he signed and internal wiki pages he wrote explaining the assumptions. Records of training sessions he ran for junior staff show that others depend on his knowledge.
A letter from the chief actuary is useful but should reference these records rather than substitute for them. Where the records are voluminous, a summary table with dates and document references, backed by representative samples, is easier for an officer to follow than a full dump. French-language records submitted to USCIS must include certified English translations.
The goal is to show the knowledge was gained through the qualifying employment, not asserted after the fact.