He can still feel emotions, just not strong ones (at least when unrelated to Miyuki)
He can like someone, but won't get to lust.
He can get annoyed, but never burst into rage.
etc.
That's why back in the chapter this detail was introduced he said he didn't 'hate' his mother for doing this to him.
He wasn't lying, because his emotion would never reach that level.
Nope, as you read on, it's more BS than you realize.
He actually felt anger when his friends got her... he feels competitiveness....
He's not as dead inside as he believes...
A huge part of his "emotion-less minus Miyuki" seems to be gaslighting than actual lesser emotions....
And he did have military and emotion control training as a child....
So he has more emotions than he himself thinks...