After rifling through one of the boxes, Carollo came up with the idea of trying to recreate his grandfather's photos during his own travels.
He's calling it the past present project, and has been posting his recreated photos on both Instagram and Facebook.
Keep scrolling to see where's he been so far.
Christian Carollo (middle) with his grandparents, mother and sister in Kennett Square, Pennsylvania in 1984
Mr Carollo holds a photo of the Golden Gate Bridge, taken by his grandfather, Herb Connellan, in 1979
Workers at the famous Cafe Du Monde in New Orleans' French Quarter (older photo was taken in 1978)
Mr Carollo's grandparents gazed at the sea when they stopped at Lucia Lodge in Big Sur, California in 1979
Mr Connellan, who died in 2008 at the age of 87, took the photos from the 1970s to the early 1990s – a time before the rise of budget flights when family road trips or staycations were the norm.
Mr Carollo calls his mission the Past Present Project, which began as a way to showcase his grandfather’s old travel snaps from around the US, mostly for family and friends.
It has now become a hit on the internet, with his blog and Instagram feed attracting attention from around the world.
Mr Carollo, a customer service manager, told MailOnline Travel he has gone through a range of emotions since the project began three years ago.
Steamboat Natchez in New Orleans, Louisiana (pictured in February 1978 and April 2014)
Mr Carollo's grandparents visited historic Knecht’s Covered Bridge in Springtown, Pennsylvania in 1994
This old US Post Office building in Chinese Camp, California (pictured in April 1979) has been shuttered
He said: ‘I was amazed that I found – and was standing in – the exact same location that my grandparents were decades before when I took the first photo [in Oregon].
‘There’s excitement when I first see a location that was more difficult to find. Joy when I see the expressions on local people's faces when I show them old photos of a place they have lived all of their lives. ‘And sadness that I cannot share this project with my grandfather and thank him for instilling a love of travel and photography in me.’
The Iron Door Saloon in Groveland California (pictured in 1979) is the oldest saloon in the coastal state
New Orleans' Jackson Square, a National Historic Landmark, in February 1978 and April 2014
Mr Carollo visited Tunnel View in Yosemite National Park to replicate his grandfather's photo from 1979
The project happened by chance in October 2012, when he got the idea after he came across a box of slides that belonged to his grandfather.
The slides contained holiday photos of the Oregon coast, which Mr Carollo happened to be visiting the following week.
He replicated a photo his grandfather took in the small coastal town of Winchester Bay 30 years ago, and decided to turn it into a project after the single image attracted interest from family and friends.
Fishermen's Grotto in San Francisco still offers the same fabulous ocean views that it did in April 1979
Mr Carollo traced his grandparents' travels in 1979 with a road trip that took him to Yosemite National Park
Mr Carollo's grandparents visited Monterey Harbor in California in April 1979; he visited 36 years later
Since then – with the help of his grandmother’s detailed travel journals to find the right spots – he has been criss-crossing the US with stops in 10 states so far and 20,000 miles behind him.
One of the most memorable moments occurred at Fishermen's Grotto in San Francisco, where he showed his grandfather's photo to a waiter.
Mr Carollo wrote on his blog: 'He immediately told me that he knew the man in the photo and, in fact, he had just finished working and was sitting at the bar.
'We met, pointed out things that had changed since 1979, and then had him stand in the same spot for this photo.'
Some locations haven’t changed much since his grandfather visited 20 to 40 years ago, other than a new coat of paint or new signs.
But a number of images reveal the slow decline of small towns, Americans’ changing wardrobes and the evolution of the car.
Mr Carollo said his grandfather inspired his love for travel and photography, and the project now often determines his next journey.
His next stops will include Salt Lake City, Grand Teton National Park and Yellowstone National Park.
Cafe Du Monde in New Orleans temporarily did away with its awning and installed a fountain in 1978
Cedar Pass Campground in Badlands National Park in South Dakota (May 1981 and October 2013)
Mr Carollo's grandparents visited the Billy the Kid Gift Shop (right) in Mesilla, New Mexico in April 1979
The photos reveal how much life has changed in nearly 40 years (pictured: Mission Santa Cruz in 1979)