Uganda's health ministry said the boy and his relatives, including his mother - a Congolese woman married to a Ugandan man residing in Kasese - had travelled to DRC before returning to Uganda on June 10.
"In preparation for a possible imported case during the current outbreak in DRC, Uganda has vaccinated almost 4,700 health workers in 165 health facilities (including in the facility where the child is being cared for); disease monitoring has been intensified; and health workers trained on recognizing symptoms of the disease", the joint statement said, using an abbreviated form of Congo's formal name. "The child is under care and receiving supportive treatment at Bwera ETU (Ebola treatment unit), and contacts are being monitored", the WHO Uganda office said in a tweet.
To prepare for the possibility of imported cases, Uganda has already vaccinated almost 4,700 health workers against Ebola and disease monitoring programs are in place, the WHO said.
Uganda has been on high alert since the start of the Ebola outbreak in DRC in August 2018.
Uganda has experienced several outbreaks in the past, most recently in 2012, while in 2000 more than 200 people died in an outbreak in the north of the country.
Health officials have feared the spread of the virus, which has festered in DRC's North Kivu and Ituri provinces for almost a year.
The health ministry said its vaccination campaign would target a further 1.4 million infants, and that 2.2 million had been vaccinated in April.