It comes as Shropshire Council says it will name one of its winter gritters after Zac.
Zac has an extremely rare form of childhood leukaemia called Near Haploid, and needs to travel for specialist treatment unavailable on the NHS.
A campaign to raise £500,000 was launched to send Zac to Philadelphia.
The amount given by Jonathan Ross has not been disclosed, but it is hoped the publicity will be invaluable. About £100,000 has been raised through JustGiving, and the total raised is now believed to be in the region of £288,000.
Ross pledged a donation to support Zac's trip on Sunday night, writing 'Good luck' on his JustGiving page.
The 57-year-old shared the page with his five million followers on Twitter, tweeting: "I've #justdontated to save Zac Oliver's life.
"Get him to Philadelphia and have a chance at life.
"Donate on @justgiving and help raise £500,000."
Meanwhile, Shropshire Council tweeted that it would name one of its gritters after Zac.
The council tweeted: "We're very happy to say that one of our gritters is to be named after four-year-old Zac Oliver, who needs to travel to America for specialist medical treatment.
"Thank you to everyone who put forward Zac's name, including his mum Hannah."
We're very happy to say that one of our gritters is to be named after four-year-old Zac Oliver, who needs to travel to America for specialist medical treatment. Thank you to everyone who put forward Zac's name, including his mum Hannah. @ZacAttacksBigC https://t.co/kDnV5hlp9h pic.twitter.com/U9sUEENFTE— Shropshire Council (@ShropCouncil) October 8, 2018
Jonathan Ross is not the only high-profile name to support Zac's cause. Other celebrity backers include footballer Jermain Defoe and Dire Straits guitarist Mark Knopfler.
Most recently, former professional footballer, Stiliyan Petrov, donated £500 to Zac's JustGiving page and wished him good luck.
The 39-year-old ex-Celtic player won 10 trophies in his time at Celtic Park before being diagnosed with acute leukaemia in 2012.
Mark Garbett, Zac's father, said it was amazing to see how far the cause has spread.
On Sunday a specially arranged festival for Zac will be held in Telford Town Park.
Zacfest will feature a host of entertainment and also see a bid to break the world record for the most people dancing along to the catchy Baby Shark song which has proved a hit on the internet.
As well as the record attempt, the day will include performances from Escala from Britain's Got Talent, The Endings, Ska Burst, Savannah and Black Bear Kiss.
Zacfest will take place at Telford Town Park from 10am until 10pm on Sunday.
Tickets cost £5 for adults, £2 for children and £12 for a family of four and will be available to buy on entry.