Who is Jason Grace's father in Rick Riordan's, The Heroes of Olympus series?

The Heroes of Olympus is a pentalogy of fantasy novels by Rick Riordan that collectively form a sequel to Percy Jackson & the Olympians. The first book of the series was The Lost Hero was published on October 12, 2010. The final entry in the series, The Blood of Olympus, was published on October 7, 2014.

Jason Grace: A demigod son of Jupiter. Jason suffers amnesia at the beginning of the book and is inclined to call the gods by their Roman names. He owns a coin that turns into a sword or a lance depending upon heads or tails, although it is destroyed during their quest. He is 15 years old and Thalia Grace's younger brother, although Thalia was born under the Greek aspect of Zeus while Jason was born under the Roman aspect, Jupiter. Jason was offered to Hera as a champion at the age of two, which later provoked Thalia to run away from home. Before the start of the book, he lived in Camp Jupiter, the Roman counterpart to Camp Half-Blood.