Kearns died attempting to save fellow soldiers, and a Finnish soldier was killed alongside him, the source said.
Shan-Le Kearns, 26, is the fourth New Zealander known to have died in Ukraine.
The Ministry of Foreign Affairs and Trade said it had not yet received any confirmation from Ukrainian authorities and was urgently seeking information.
However, it said the process may take some time given the situation on the ground in Ukraine.
It said no further information could be shared, citing privacy reasons.
Elis Kearns told 1News her son was not on the front line but had been trained in combat so he could help rescue injured soldiers.
He had been in Ukraine for two years and was intending to stay for another to fulfil his three-year contract, she said.
Kearns is the fourth New Zealander known to have died in Ukraine. Dominic Abelen, 28, was killed on August 23, 2022; Andrew Bagshaw, 47, on January 6, 2023; and Kane Te Tai, 38, on March 20, 2023.
The Weatherman Foundation is an American NGO which has been in Ukraine since the start of Russia’s full-scale invasion of the country.
Kyiv-based Lauren Guillaume, who is from the foundation’s missing-in-action programme, said it started out delivering humanitarian aid but had developed “to really cover the life of a soldier – all the way from treating the wounded in action both their physical wounds and mental PTSD wounds”.
The programme also supports the families of the missing in action and killed in action, she said.
“Unfortunately these days the difference often between a missing in action and killed in action soldier is whether or not their body can be recovered from the battlefield,” she said.
“What our team does is investigate to identify their body and we also pursue a court case to help the family try to receive a death certificate in the terrible circumstance if a body cannot be recovered.”
The only way a family such as Sharn-Le Kearns’ can obtain a death certificate “is either through his physical repatriation or through a court case”, she said.
“So we are helping them in this process all the way from collecting DNA back in New Zealand, helping them to navigate the Ukrainian judicial system by helping them to collect witness testimonies and submit a case to the Ukrainian court, all the way to eventually helping them apply for the compensation that they’re owed.”
The foundation’s team also visits morgues all around Ukraine to look at unidentified remains on the chance that it could be him, she said.