Though he did his best to care for his flying seahorses, they only continued to get more and more sick until his mom had to call the old man and ask him to make a special house call to check on the fish personally.
Unbeknownst to them, a powerful typhoon (Typhoon Dianmu) was approaching. It hit their area around the time the old man was to be on his way. Though they hoped he'd decided not to come given the sudden weather, he still tried to get there but ultimately died in the storm, due to the ferocious winds and rainfall of the typhoon.
Because the old man wasn't able to make it, Makoto's flying seahorses subsequently died a few days later. He's 7 years old at the time.