Complex sentence: Between 1875 and 1885, his health declined by a paralytic stroke and heart disease while subsequently struggling with business as timber became scarce and taxes and immigrant population increased so he invited his son to the business in 1880.
Split into simpler sentences: Between 1875 and 1885, his health declined when he suffered a paralytic stroke and heart disease. At the same time, he was struggling with his businesses as timber became scarce and taxes and the immigrant population increased, so he invited his son to the business in 1880.