They have released an e-fit of a man they would like to speak to in connection with the robbery, which took place in Huddersfield on March 28.
The robbery happened on New Hey Road, at around 8.30am that day, when a man, described as being about 40, approached the 13-year-old victim and punched him in the stomach before stealing his crutches.
The suspect is described as being around 6ft 2ins and muscular, with a bald head and a white and brown beard. He was wearing a black coat and navy blue jeans.
Information can be passed to Kirklees district police via 101, quoting crime reference 13180146968, by using the 101 Live Chat facility on the West Yorkshire Police website or by calling Crimestoppers, anonymously, on 0800 555 111.