What else can I do?!?

My little is 3 years old he was 3 last November. I tried potty training him a couple of times after his 2nd birthday and he had no interest at all. When it came to his 3rd birthday I was determined we was going to  and we were going to bin the Nappies, I've tried bribery with food, weve made a special 'potty sticker book' together, I've taken him shopping with me so he can pick his own big boy pants. But we're still stuck in the mud he's never been to toilet he constantly wets himself, and tells me or someone at his pre-school after he's done it, I tell him not worry about it just tell me cause he has big boy pants on now. This has been going for months now! I'm literally in tears everyday cause he is the one at his pre-school still in nappies. I feel useless as a parent like I'm doing something so wrong. He's my only child and I've just found out I'm having another baby I don't know if I'm doing so wrong with his potty training or I'm just putting to much pressure on him. I'm completely stuck on what to do but I don't want him to start school and him still being nappies I'd feel like even more of failure then. 

Any help or advice would be so appreciated. I'm at my wits end and I really don't know what to do anymore. 

Thank you.