Brian White, from Wolverhampton, moved to the UK from Botswana aged 15 with his adoptive family.
An application for naturalisation was rejected and he now fears being returned to his country of birth.
The Home Office said it had reassured Mr White it was trying to resolve his application as soon as possible.
A spokesman added it understood the "urgency" of his case.
A petition highlighting his battle has been signed thousands of times and received celebrity support on Twitter.
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Mr White, 21, lived in a Zimbabwean orphanage until the age of six. He was fostered and later adopted by the White family, who moved to Botswana and later returned to the UK.
In 2014, his application to become a British citizen by naturalisation was rejected as when he arrived in the UK, he was granted limited leave to remain by the Home Office, rather than indefinite leave which is required.
The family appealed, but that was also rejected. He is now awaiting the decision of a current application.
He was unable to take up his place to study chemistry at Oxford in 2016 after being deemed ineligible to receive student finance.
Mr White said the university kept his place open for him for a year, but it could be in jeopardy if his application is not approved.
"I don't think I've ever had time to sit down and think about it," he said.
"I'm trying my best to get it out of my mind."
He added he was "astounded" at the support of the petition, which has been shared by the likes of journalist Caitlin Moran and writer Philip Pullman and signed over 4,000 times.
End of Twitter post  by @PhilipPullman
End of Twitter post  by @caitlinmoran
Friend Luke Wilcox, who set up the petition, said it would be "disgusting" if the application was rejected.
"It's terrifying. He has got no links there, no family, no friends," he said.
A student faces being deported and losing his place at Oxford University due to uncertainty over his citizenship status.