Why in the world would you agree with him on that? You don't think newborn infants and certain disabled people are human beings?!
I will have to check the specifics about infants, but indeed, certain disabled people are not human beings far as euthanasia goes. In any case, that is hardly the only factor to consider. There are situations when euthanasia is needed to diminish suffering.
Myself, I do not think you know half as much about Utilitarian Ethics as you believe you do.