It is also believed he may have black sunglasses with him.
He was first reported missing in Tunbridge Wells, Kent, on Tuesday 20 April and travelled to Berkshire.
Detective Inspector Roger Lamboll, based at Abingdon police station, said: “Doug’s disappearance is very out of character, and it has now been a month since we have had any confirmed sightings of him.
“Today, we are releasing two new photographs of Doug and are again appealing for the public’s help in tracing him. His family and friends are naturally extremely concerned for his welfare.
“We think when Doug left his vehicle on The Ridgeway, there was a possible sighting of him leaving on foot in the general direction of Aldworth, but there have been no confirmed sightings of him.