Nathan Grainger was last seen at his home in Surrey Street, Brighton, at about 2pm yesterday, when he said he was going to cycle to Rottingdean.
He has been known to stay out late, but never overnight and there are concerns that, due to learning difficulties, he has a reduced sense of danger and is reluctant to engage with people to ask for help if he has become lost.
Police are considering whether he may have continued his journey on to Eastbourne, or, headed west towards Worthing, where he has recently enrolled at college.
A South East Air Support Unit helicopter has joined the search along the coast.
Nathan is white, of slim build, 5ft 8in, clean shaven, with short dark brown hair and wearing denim shorts, a black hooded top with blue polo shirt underneath and black Adidas trainers.
He is riding a red Raleigh hybrid bike with black handlebars that are curved inwards.
He does not wear a helmet.
Anyone who believes they have seen Nathan is asked to call 0845 6070999 quoting serial 560 of 27/08.
A Sussex Police spokesman said: “If he won’t be approached, is in danger or requires medical attention, please dial 999 immediately.”