The rim was full of dirt and sand so my son had to clean the rim using a brass wire brush and a scraper before installing the tire.
After having practiced with lots of bicycle and motorcycle tire replacements, he seems to be getting better doing it. He just hates replacing off road tires especially trials and motocross competition tires because the tires are harder compared to normal tires.
He had a problem getting one part of the tire's side to seat in the rim. First he tried using water and soap but it didn't work so he tried using tire cream but still it didn't work. Then he used deep penetrating oil (WD-40 / CRC) and finally got the tire sides to sit in properly on the rim.
He looks happy that he finally got it right.