The Broseley five-year-old. who has battled a rare type of childhood leukaemia. has been in Philadelphia for his nine-month post operation biopsy and other tests.
All results came back negative, another boost for Zac and his family after he was given the all-clear earlier this year.
The smiley youngster, who stole the hearts of the Shropshire public in his battle, enjoyed a trip to see the dinosaurs at the Natural History Museum, and even got to sit in the cockpit of the plane on the way home.
His mum Hannah Oliver-Willetts said in a heartwarming post on Instagram: "Zaccy had a busy week. We both flew out to Philadelphia last Sunday for his biopsy. He's been getting headaches for a few weeks so we were all very worried about the results. However we were worrying for no reason because all his results came back negative.
"A charity came to the children's hospital to make pillow cases with the children. Zac chose crocodile print material. He has slept on it ever since.
"We went to Zac's favourite place, the Natural History Museum (dinosaur museum). He absolutely adores the place.
"On our way back, the pilot asked Zac if he wanted to go up front to sit in the cockpit. Zac was in there like a shot."