Job titles (which are often just descriptions of a person's job anyway) are only normally capitalised within the confines of an employing organisation and only when referring to the post itself rather than the person occupying it. Even then, it is a stylistic choice and not a 'rule' of English.
- I work as a security guard.
- The company has a security guard
- We have an opening for the post of Senior Security Guard.
In your example, there seems no need to capitalise such a generic job title as 'security engineer'. Personally, I don't know what a 'security architect' is, and having it capitalised makes it stand out as a job title. But some might consider the inconsistency of having one capitalised and the other not to be odd. It might be better to say "he held the post of Security Engineer" so that you can legitimately capitalise both.