There are two Theodore Notts in Book 5, and neither of them appears for very long. There is Theodore Nott, Sr., who is a Death Eater who takes part in the ambush at the Department of Mysteries. He gets injured in the proceedings, and Lucius Malfoy shouts, "Leave Nott, leave him, I say – his injuries will be nothing to the Dark Lord compared to losing that prophecy" (35.84).
Then there is Theodore Nott, Jr. He is a son of Theodore Nott, Sr. and a Slytherin. He is a "weedy-looking boy" (26.112) who is clearly furious that Harry has publically named Nott, Sr. as a Death Eater. But, his father is a Death Eater, so his protests seem a little pointless.