Matthew Collett-Leslie, 14, from Hastings, was first referred to Sussex Child and Adolescent Mental Health Services (Camhs) in November 2014.
In May, a psychologist said he showed signs of autism but would need a full assessment, scheduled for 26 November.
The NHS trust said there had been a significant rise in referrals.
Matthew, who was referred to Camhs by his school, Hailsham Community College, has started his petition on charge.org and written to the prime minister.
He said the wait for a possible diagnosis had affected his school and home life.
"Children do pick on me sometimes because they don't feel the way I am feeling," he said.
"There is not much support at school because I am not officially diagnosed."
His father Joe said Matthew needed help and support at school from a teaching assistant.
"At home I need the support for myself to deal with the situation," he said.
Tom Purser, of the National Autistic Society, said a diagnosis of the condition could help young people understand their behaviour and entitle them to support at home and at school.
Sussex Partnership NHS Foundation Trust said: "Assessments for autism involve several stages with a number of different professionals because the symptoms are complex and often confused with other conditions.
"There has been been a significant rise in the number of referrals to us which has added to the pressures on our services.
"It is a key national priority to reduce the wait and we are working with other NHS professionals to look at the delays... to see what more can be done."
Phil Matthews, principal of Hailsham Community College, confirmed that the school made Camhs referrals for students when considered appropriate.
A teenager with suspected autism who faces a wait of a year for a mental health diagnosis has started a petition to cut waiting times.