He was assaulted and had his phone stolen near Regent Road, Great Yarmouth, Norfolk.
Officers were called shortly after 03:30 BST and the scene has been cordoned off while inquiries were being carried out.
The force said the man was being supported by specially-trained officers. Police appealed for anyone in the area at the time to contact them.

Garry Barlow, 49, from Liverpool, was one of six Britons killed by Islamist militants in the siege.
Lorraine Barlow told the BBC she had been given no information about the progress of the investigation or whether there was sufficient security.
The siege ended after Algerian forces stormed the site.
Mrs Barlow first learned about the attack on the gas facility when her husband called and said he had been taken hostage by extremists who had strapped explosives to his body.
She had two more increasingly frantic calls but was later told he had been killed.
In an interview with the BBC, Mrs Barlow said she was shocked and distressed at the attitude of BP - which is part of the joint venture that runs the site - after her family were not invited to a memorial service.
The company said her husband was employed by an agency and not BP.
In a statement, BP said: "The terrorist attack at In Amenas was an unprecedented and horrific criminal terrorist act which has profoundly impacted the lives of many people.
"Our thoughts and sympathies remain with the families of all those who were killed.
"The In Amenas plant is run by a joint venture between Sonatrach, (the national oil company of Algeria), BP and Statoil.
"The joint venture has its own management and operates independently. Some personnel are seconded by the shareholders to the joint venture, which also engages contractors and international agency personnel directly, including through Garry Barlow's Swiss employer IOTA."
The investigation into the attack is being led by the Algerian authorities.
Officers from Scotland Yard's anti-terrorist squad have been to Algeria to gather evidence for the British inquests.
A lawyer representing 30 bereaved relatives and survivors of the attack said they feared the process may not address the wider questions about how their loved ones died.
Forty-eight foreign workers were killed following the siege at the In Amenas plant.
Twenty-two British survivors were flown back to the UK and reunited with their families, while Algeria said 29 militants were killed and three captured alive.
Mr Barlow was a system supervisor at the In Amenas plant.

The fire at Shorrocks Hill Country Club in Lifeboat Road, Formby, was discovered at about 05:40 BST.
A joint investigation with Merseyside Fire and Rescue Service is under way into the cause of the blaze, Merseyside Police said.
The dead man is believed to be Andrew Coogan, although formal identification has yet to take place.
Firefighters said he was found with the horses in a stable which destroyed in the blaze.
A man has been raped and robbed in the early hours.
[NEXT_CONCEPT]
The widow of a British hostage killed in an attack on an Algerian gas plant in January has said she still does not know how her husband died.
[NEXT_CONCEPT]
A man's body has been found by firefighters tackling a blaze that killed a number of horses.