Twin Signal (1992)

Blurb:
Twin Signal begins with the 11 - year-old Nobuhiko Otoi going to the countryside (Tokkari Town, to be exact) to live with his grandfather, Professor Shinnosuke Otoi. His parents have gone to work/research in another country, but they do so much traveling all the time that they decide to leave Nobuhiko with his grandfather on a permanent basis. Once there, Nobuhiko finds out that his grandfather has made a new fighting HFR called Signal. He persuades the Professor to reprogram Signal from being a battle robot to being his older brother and friend. During the programming, an accident occurs resulting in Signal transforming into Chibi Signal whenever Nobuhiko sneezes, but Signal’s aggressive battle programming wasn’t changed yet so he is somewhat scary by Nobuhiko’s standards.

Source: www.anisearch.com/manga/2772