The guy was not breast feeder when his father got a new mother for him. Can he get married to the sister of the new mother? the boy was matured when his father got married to this lady)
Yes a man can marry the sister of his step mother, but could never marry his step mother as she is a mahram because of (4:22) which make all wife's of the father haram to his sons,
And do not marry those [women] whom your fathers married, except what has already occurred. ...
And [also prohibited to you are all] married women except those your right hands possess. [This is] the decree of Allah upon you. And lawful to you are [all others] beyond these, [provided] that you seek them [in marriage] with [gifts from] your property, desiring chastity, not unlawful sexual intercourse. ...
-of course with details which could be find in ahadith (for example aunts of the wife, who are made haram for the husband unless he is legally separated from his wife)-.
As sisters of a step mother are non-mahram scholars say that the step son shouldn't meet them without a mahram and they shouldn't appear unveiled in front of him unless he was breast-fed by his step-mother!
But generally a mahram is either by alliance (marriage: mother, step-mother, grand-fathers-wife, wife's-mother or grandmother, daughter of a wife, son's or grandson's wife), lineage (ancestors like father mother grandfather grandmother, descent daughters, sons, grandchildren) and by breast-feeding.
And Allah knows best!