Piers Hopson, aged 35, left his home in St Leonards-on-Sea on Monday (January 25) at around 2.30pm, saying he was going for a walk but did not return home.
Roger Hopson, Piers' dad, was in Eastbourne on Wednesday (February 3) distributing flyers.
Roger, 65, told the Herald, "Eastbourne is one of the places he knows as he used to go to college here years ago, so it is somewhere which is familiar to him.
"He is very vulnerable. He has autism and Asperger's syndrome and his social skills are exempt.
"He would not ask for help. He would pace up and down and expect someone to come up to him.
"If you ask him his name he will tell you. If anyone in Eastbourne does see him go and speak to him and then if you have a mobile, phone the police and stay with him until the officers arrive.
"So many people think they see him but they return home and think about it for 45 minutes before calling the police."
Piers is known to like walking and has previously walked to Eastbourne and Heathfield.
Roger added, "A person can get a long way in 45 minutes, so if you see him please stay with him."
He also explained that Piers may now have a beard and look unkempt as he has been missing for some time.
He added that he and his wife Dawn Hopson were very worried about their son and are hoping that he has been befriended by someone.
A police spokesperson said, "If you find Mr Hopson and he is unwell, please call 999 immediately to request police and ambulance attendance.
"If he is safe and well, please contact Sussex Police on 0845 60 70 999, quoting serial 1721 of January 25."