There are several pieces of evidence for this:
1. Kid Akainu already had a marine cap(did his father give him the cap?)
2. The admiral's murder hasn't been shown in this flashback, but it would be odd if Oda didn't show it. It's most likely that he'll show it later, in Akainu's flashback.
3. Oda seems to like giving important characters powerful fathers.
1. Kid Akainu already had a marine cap(did his father give him the cap?)

2. The admiral's murder hasn't been shown in this flashback, but it would be odd if Oda didn't show it. It's most likely that he'll show it later, in Akainu's flashback.
3. Oda seems to like giving important characters powerful fathers.