Holmes said the boy's quick-thinking father put his son straight into a shower of cold water.
"He did everything right. You smother the flames or roll them in a blanket and then put them in cold water for 20 minutes."
Holmes said the boy was wearing loose-fitting clothing, possibly a t-shirt, which caught alight.
"He's either come into contact with the heater itself or he's been sitting too close to it for a short period of time and the clothing ended up on fire."
He said anything combustible, whether it was clothing, furniture or a person, needed to be at least one metre from any type of heater including fires.
"It's a horrible accident. Just a terrible, terrible accident. You wouldn't wish it on anyone."
The boy was flown to Waikato Hospital by the Westpac Waikato Air Ambulance last night. His family is at his bedside.
Holmes said the boy had only started school this year and was probably too young to have seen the New Zealand Fire Service's Get Firewise programme which is taught to primary school children.
He said if there was anything to be learned from the tragic accident it was to "keep an eye on your kids around heaters".