my 4yr old son wont wee on the toilet

i have a 4 yr old son who will not use the toilet...he doesnt like wearing nappies and insists on wearing big boy pants.he knows when he wants to go a wee and is very confident in going up to the toliet but then that is it, he just doesnt go!!! he stands there for ages but wont go.ive tried getting him to sit on the toilet and he will just sit there,same thing nothing!! ive tried sticker charts etc buying toys,getting him a gameboy advance to play while sitting on the toilet but nothing has worked...then after alot of time and fuss he will go bk downstairs then 5mins later wets himself....why?? i dont understand.his brother is 5 and i was hoping he would copy him but no.i bought a potty and the same thing there he doesnt go...and he knows when he wants to go so why doesnt he? really need some answers please...