What could be the reason for his return and what can be done to manage such situation?
The only reason I can think of is that you can now handle yourself well at the betting table, know when and how to stop, and fully manage your limitations. If you’re going back, it's a good thing. However, if you’re returning to try to make money and remember your winnings, only to try again, you'll likely mess everything up and may end up worse off this time.
You can go back if you are already good at managing your gameplay, as this allows you to avoid the traps you are in.
Indeed, if you can't control yourself better, it's best to never think about gambling again, especially after having quit for so long. If your goal is still to win and not to gamble for entertainment purposes, then I think it's better not to go back, as it will surely lead to bigger problems than before.
It's difficult to find a definite reason why someone who has quit might go back to gambling, it could be due to peer pressure or being tempted by the big wins of others. Ultimately, it doesn't matter if you go back to gambling and feel like you are a better version of yourself by having boundaries that make you act more responsibly.