A light/web novel about a person reincarnated as the youngest child in a noble family, who later learns magic from a zombie magician
I found it. It's Hachinan tte, Sore wa Nai Deshou!
Synopsis from mangaupdates:

Ichinomiya Shingo, a young 25 years old, single, worker of firm company while thinking of tomorrow' busy working day goes to sleep. However the moment he woke up, it's a room unknown to him. He then learn that he is inside a 6 years-old boy and is taking over his mind. He then learn a lot from the memories of the said boy: he was born as the youngest child (8th son and 10th child) of poor noble family living in back country. Having no administrative skill, he can't do anything to manage the vast land his family has.
Fortunately, he is blessed with a very rare talent, talent of magic. Unfortunately, while his talent could bring prosperity to his family, in his situation it only brought disaster. Yes, this is the story of the boy, Wendelin Von Benno Baumeister, opening his own path in a harsh world.