The Irishman, 26, had been told that he could leave Brisbane Road at the end of last season but has won Ling over with a strong start to the new campaign.
Ling has been impressed with the player's attitude in training, and now sees him as having a bright future at Brisbane Road.
"JJ has come back to pre-season with the bit between his teeth and he's looking like the player I signed last summer," he told the Epping Forest Guardian.
"When I spoke to a lot of people at the time they said I'd got a steal getting JJ on a free from Cheltenham because he was playing really well.
"JJ still has two years left on his contract and hopefully we can start to see the best of him now."