Spoiler.
To make it short, he's not. The child that look like ml apparently is his older brother from the same mother. Their mother keep the brother hidden because of the DID. So, he's a full grown man that also the owner of gambling house where the half brother has his debt is (inherited from his late mother). One day this bij hit the owner with her car, and that accident trigger him to split into new persona + he lost his memory. And with this chance, the half brother plant a new memory/persona that called Felix: ml's bastard son. He's killing two birds with one stone: ruining ml then claiming fl as his and erasing his debt by "killing" the gambling house owner & the business.
I've started reading this so long ago that I cant remember anymore, did he cheat on the FL? I feel i have holes in my memory when it comes to this story but im too lazy to read everything again from the beginnig so I would be grateful if someone reminded me (▰˘◡˘▰)