Vadim Kobzev said Navalny, who is in a penal colony serving a sentence for embezzlement, had been diagnosed with two spinal hernias.
Last week, Navalny started a hunger strike to demand proper treatment for acute back and leg pain.
The White House said reports that his health was worsening were disturbing.
Mr Kobzev, who visited Navalny on Wednesday, said on Twitter (in Russian): “Alexei is walking himself. He feels pain while walking. It is very concerning that the illness is clearly progressing in terms of losing sensation in his legs, palms and wrists.”
Earlier this week, Navalny, 44, was moved to a sick ward in the prison in the town of Pokrov with symptoms of a respiratory illness. He had complained of a persistent cough and temperature.
In an Instagram post, Navalny said prison authorities were trying to undermine his hunger strike by roasting chicken near him and placing sweets in the pockets of his clothes.
Also on Wednesday, White House Press Secretary Jen Psaki said the Biden administration considered Navalny’s imprisonment “politically motivated and a gross injustice” and called for his immediate release.
Earlier, human rights group Amnesty International said Navalny had been incarcerated in conditions that amount to torture and may slowly be killing him. Campaigners say the prison is known for its especially harsh conditions.
Navalny’s lawyers, who have been visiting him in custody, say there are no doctors at the colony and the medical unit on-site is run by a single paramedic.
Last week, Russia’s prison service denied Navalny’s allegations that he was not receiving proper treatment, saying he had been given “all the necessary medical assistance in accordance with his medical indications”.
Navalny was given a suspended sentence for embezzlement in 2014, a conviction widely seen as politically motivated.
The suspended sentence became an actual prison term in February, following his return from Germany, where he had been receiving treatment for poisoning with the nerve agent Novichok.