Potty trainning HELP!!!!!!!!!!!!?

Two and a half year old son goes everytime when naked as soon as any kind of clothing goes on him he screams for a nappy then doesnt even attempt toilet and wets himself he is very inteligent at everything else and 100% understands me he just wont do it! Weve had a break from it for apprix 2 months now due to a new baby but we really need to get going again now any tips or advice would be greatly appreciated ive tried kiddie books and dvds about potty trainning a reward chart treats when he goes and praise he never gets wrong for it! So i dont know where to go from here......he goes to nursery in january so i need to get this nailed..... thankyou