Adam George, 11, claims Flambards Theme Park, in Cornwall, discriminated and failed to make reasonable adjustments for him under the Equality Act.
Changing places toilets are required by an estimated 250,000 people in the UK who need extra specialist equipment.
Flambards says it takes inclusion of all visitors seriously and has made adjustments to its toilets.
The company says a permanent changing places toilet would cost over £40,000 – an expense that could affect jobs.