IRCC is probably running background checks. There are a number of ex-defense services applicants who have applied in August 2016 (and later) and are still waiting for decisions.
Your dad's application could be taking longer to process if:
1. He held an operations-type role. For example, non-administrative functions/departments
2. He was posted in border areas (non-family type postings).... worked in high-security areas..... handled highly-sensitive information.... served during a war
3. He was working at a senior rank in the defense services
4. His travel history did not include countries like the US/UK/Schengen, etc
Here's the other thread:
http://www.canadavisa.com/canada-immigration-discussion-board/trv-for-retired-army-officers-from-india-t457534.45.html
TRV decisions in these cases are taking several months beyond the processing times stated on the IRCC website
Cheers