Six-year-old Rhys Harris of Newbridge, Wales, was discharged on Saturday after a successful bone marrow transplant.
He is one of only about 40 people in the world with Nemo, a genetic disease crippling his immune system.
He has spent months in a sterile bubble at a hospital in Newcastle because he could not fight off infection.
The bone marrow will help him build a new immune system.
Rhys, his parents Kevin and Dawn and younger brother Morgan moved from south Wales to Newcastle, where the treatment took place, after a worldwide search for a donor.
The family have already been able to enjoy a day trip to the seaside from the hospital.
Harris said: "It's earlier than ever we thought we could come out with him - it's great to see him running about, and he really needs to strengthen his feet and start his muscles working again.
"He's seeing all these things he can't remember before, like buses, Christmas trees in people's windows - he sees a dog and he smiles, it's precious."
"We have to be very careful about where we go with him and for how long, but it's nice to get out in the fresh air after so long," Harris said.
The donated bone marrow was flown from America in October before the transplant took place.
The youngster's immune system had to be destroyed by chemotherapy before the bone marrow transplant could take place.
Harris said, in his online blog, "We are all just trying to get into a normal rhythm again now. Rhys has a whole pharmacy of drugs to contend with, but it is not too bad.
"He is, to put it mildly, mad. He has not stopped, beating on his brother, running around like a lunatic. Hopefully the novelty of being home will wear off soon and he will slow down."
Rhys, who is deaf following a bout of meningitis, will have to live in a special germ-free room for 12 months and have restricted contact while his immune system rebuilds.