Massimo Galioto, from Rome, was arrested for manslaughter, police said in a statement.
Solomons body was found in Romes Tiber river, near Ponte Marconi, on Monday morning, three days after he went missing.
Galioto's companion, with whom he shares a tent on the embankment, told Italy's Rai Uno he had not meant to hurt the student.
A drunk Solomon had "bumped into Massimo and they began to argue. They were pushing each other. Massimo pushed him, the boy pushed back, and then he fell in the water," she said