Complex sentence: Upon the restoration of the Spanish monarchy in 1975, Juan's son Juan Carlos became King and awarded his father, who had renounced his rights to the throne, the title of Count of Barcelona in 1978, which he held until his death in 1993.
Split into simpler sentences: Upon the restoration of the Spanish monarchy in 1975, Juan de Borbón was passed over as king in favor of his son Juan Carlos. After he renounced his rights to the throne, his son officially awarded him the title of Count of Barcelona in 1978, which he held until his death in 1993.