Well beside the normal riding gear, We carry Waterproof's, Spare clothesand shoes to change into after the trial, Food usually chocolate, nutri grain bars, crisps and energy sweets. Drinks see what he likes We carry a Red bull like drink, 4 Powerade's and 2 litres of water. We also have some wipes and latex gloves. Don't forget you'll need a decent jacket, boots and waterproof's yourself.
Then for the bike, Spare fuel already mixed, Chain oil, WD-40 a mix of spanners, screwdrivers, hex spanners etc. Spare plug or two some ty-wraps and gaffer tape. Any thing else you'll pick up as you go. Which trials are you attending??