The boy's dog bit the suspect who then stepped back.
But as he fled, he caught the youngster with the knife leaving him with a cut to his face.
The suspect - who was in his late teens to early 20s - was wearing a black beanie hat, a black neck scarf which covered the lower part of his face, a black Helly Hanson waterproof jacket, tracksuit bottoms and black gloves.ã€€
Police issued an appeal for witnesses today following the incident, which happened around 5pm on February 16.
Anyone who can help should call 101.