A prestigious Belgian mathematics professor is leaving New Zealand after his autistic stepson was refused residency on health grounds.

Associate professor Dimitri Leemans said Immigration New Zealand had broken the UN convention on human rights by refusing his 13-year-old stepson Peter residency because of the ongoing and future health burden of his autistic condition.

Leemans will not appeal the decision and will return to the Université Libre de Bruxelles in Belgium in July with his family.

“INZ took already 18 months to reject our application and we did not feel like waiting another year for the decision on the appeal,” Leemans said by email.

“I do not see myself raising my children in a country that does not respect the UN convention on human rights.”

Immigration New Zealand said in a statement that Peter was deemed to require “continuous, structured residential care”.

“All migrants are required to have an acceptable standard of health so as not to impose undue costs and/or demands on New Zealand’s health and/or special education services,” the agency said.

“An independent medical assessor carefully considered the family’s submissions but reached the decision that Peter did not have an acceptable standard of health.”

Leemans emigrated to New Zealand with his wife and two children to take up a position at Auckland University in 2011.

He said the initial reason his family chose to move to New Zealand was because they were under the impression it was a good country to raise a child with autism.

In 2014 Leemans was awarded the New Zealand Mathematical Society research award, and has also been the recipient of a NZ$580,000 Marsden grant while in the country.

Astrid an Huef, president of the New Zealand Mathematics Society, said the departure of “an outstanding mathematician” like Leemans would be “a great loss for us and New Zealand”.

“The New Zealand mathematics community deeply regrets that we are losing our valued and talented colleague, Dimitri Leemans, under such sad circumstances. We have benefited greatly from his research expertise and teaching excellence.”

University of Auckland vice-chancellor Stuart McCutcheon said Leemans was “an example of the outstanding young academics we must have at the University of Auckland if we are to maintain our premier world ranking”.

New Zealand media was scathing about the government decision. One columnist wrote: “We can’t afford to write off kids with special needs. They might be the ones who will solve some of our biggest problems. Oh, and give us joy.”

Autism New Zealand’s chief executive Dane Dougan said it was seeking further information and clarification on the ruling by Immigration New Zealand.

In the year 2014/2015, Immigration New Zealand declined 116 residency applications solely on medical grounds.

The disability rights commissioner, Paul Gibson, was not available for comment.