Will the son be rewarded for his dutifulness to his mother and helping her monthly, despite his feeling of aversion and lack of love towards her, and is he considered negligent in his dutifulness to her?

It does not appear that you are falling short in fulfilling your mother's rights. Your discomfort with her treatment is not considered disobedience (uquq) as long as you continue to treat her kindly and with beneficence. Allah Almighty says: "Your Lord is most knowing of what is within yourselves. If you should be righteous, then indeed He is ever Forgiving to the oft-returning [to Him]" (Quran 17:25). This includes any unintentional gesture or slip from a child towards their parents, as Allah forgives those who are righteous and sincere in their intention to show kindness.