@UltimateAdrian20 well, if the author wants to go for full fledged drama trainwreck, then it could be possible that Sensei's Onichan, got an older lady (handas mom) preggo, and she not wanting to screw up the kids future just let him go.
@_Happy_ If the mom just got pregnant and not married, then it would fit.
@neorasp we are of the same mind here.
@ jokerxhisoka he only mentioned he inherited his eyes and hair color from his dad, he hasnt mentioned anything about knowing or not knowing him, and if his mom is at the hospital and he has had to keep a job to pay the bills, then its obvious that Dad hasnt been around for a long while now.