It would be a surprise if your son's dirt bike has a smaller motor than a moped.
From the stltoday.com
The cost of a moped is about the same as a years zone 1-4 travelcard in London.
From the guardian.co.uk
We moped about it a little bit, but we got excited about it for the most part.
From the denverpost.com
The Downend School pupil died following a moped crash on Charlton Road, Kingswood.
From the thisisbristol.co.uk
Not cheap, but decent quality and way better than anything that arrives via moped.
From the guardian.co.uk
A pair of women rode up on a moped and scooped up a sizable share of the lot.
From the washingtonpost.com
Don't assume your auto insurance covers your use of a motorcycle, scooter or moped.
From the sfgate.com
You'll feel it as you ride a rental bike or moped on the residential streets.
From the kansas.com
He moped for a good while, and, although he's quiet by nature, he seemed withdrawn.
From the dailyherald.com
More examples
A motorbike that can be pedaled or driven by a low-powered gasoline engine
Mopeds are a type of low-powered motorcycle designed to provide economical and relatively safe transport with minimal licensing requirements. ...
Moped was a New Zealand band formed in Palmerston North by three brothers, Gareth (guitar & backing vocals), Hayden (bass and lead vocals) and Karl Shearman (drums). They played a brand of groove saturated reggae rock, with lyrics which speak of a clear Christian commitment.
A lightweight, two-wheeled vehicle equipped with a small motor and pedals, designed to go no faster than some specified speed limit; Melancholy, dejected
(Mopeds) Small, light, inexpensive, efficient rides for getting around town. Usually started by pedaling (motorcycle + pedals = moped).
Stupid, melancholy for want of society.
A two-wheeled motorized bicycle (short for motorized pedal bicycle) with an engine of less than 50cc with maximum speed of 45kmh
A scooter with a 50cc engine whose top speed is restricted to 31 mph.