potty training - little success

Hi, im in great need of some advice. We are currently in the process of trying to potty train our almost 4 yo son who has cp. We dont seem to be having a great deal of success and im at the stage now where im not sure if we are doing the right thing by trying this now. C is quite happy and understanding of the fact that babies wear nappies and he's a 'big boy' so needs pants but seems completely oblivious to the fact that he needs to use the potty until its too late. He quite proudly says'Mummy look' when he's wet his pants but then thats too late! We went straight from nappies to pants as we felt that he would need a very obvious difference in his underwear - which he fully understands and embraces but still he isnt managing to not wet himself. The only time we have success on the potty is if we time it right by asking him to try -if he does go we follow always with huge praise and C is hugely proud in his achievement. I would really appreciate any advice or direction from someone who has been through this and come out the other side- im really struggling to stay calm and positive about the endless accidents we keep having and have honestly run out of ideas! Thanks