The former Boro goalkeeper helped Liverpool into the FA Cup final yesterday.
Boro academy chief Dave Parnaby said: "When he came here, he was raw and inexperienced but very athletic. We soon realised he had talent and that he was as brave as they come, which is essential for any keeper. Stephen worked hard to bring the best out of him.
"He's a confident, laid-back lad with a strong character, who I'm sure will take Wembley in his stride. He won't let anyone down and I'm sure he'll be up for the challenge. We wish him all the best."