Lucy Baxter says she will even pay for a prostitute so he can lose his virginity.
The mother has now set up a Bebo page for Otto appealing for potential dates to come forward.
The leading campaigner has three other adopted sons with Down's.
She said: "I'd like all my boys to find love and enjoy sex.
Otto Baxter
Otto is heading for Las Vegas
"I always look at what other people are doing and why shouldn't they do the same things?
"I strongly believe, and have always said, that society has a learning disability when it comes to Down's syndrome.
"If he doesn't get a girlfriend, I will feel really bad, because I have sold him this thing that he is like everybody else. That's why I'm working overtime to get this sorted for him."
Otto enjoys acting and has appeared in local stage versions of Macbeth and The Canterbury Tales.
He persuaded his mum to help him find a girlfriend after being rebuffed for three years - and says presenter Fearne Cotton is his ideal woman.
He said: "I'm on a mission to find a girlfriend. I'm looking for girlfriends everywhere."
Miss Baxter, 50, of Abingdon, Oxfordshire, has never married but has four adopted sons - James, 25, Otto, 21, Titus, 14, and Raphael, seven.
She said: "Everyone else his age is having sex and enjoying being young adults, so why shouldn't Otto?"
Down's Syndrome: Otto And Lucy Baxter In Sex Story - Mother Appeals For Girlfriend For Adopted Son | UK News | Sky News
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