trousers (dark if he has light legs and light if he has dark legs) shirt,tie and waistcoat/tweed,gloves and a hat of some kind (doesnt need to be a riding hat, just a flat cap or trilby)
If it is an in-hand class (and at a fairly low level i assume as it is your first show) i would recommend wearing the same as if you were to be in a ridden class- smart light coloured jodhs, shirt, stock, tweed or plain jacket, riding boots, hat and gloves. Alternatively you can dress in a smart skirt/ trousers and short but i always think riiding gear looks better