In my opinion it sounds as if he isn't quite ready yet, especially with sticker charts and treats not doing the trick.
Maybe first try with some familiarisation. Like keep him in nappies most of the time. Before showers and baths see if he will use the toilet. IF he does do a wee make a HUGE deal out of it. Let him come in with you and his daddy or see other little children use the toilet/potty. As soon as Ryan saw his little cousins using the toilet he wanted to be just like them. I think there are also some great books around for young toddlers to grasp the concept of using the toilet.
When we first started toilet training i would take him to do a wee every 45 minutes. I wouldn't even ask him, just took his hand and said come on lets skip to the loo, (or jump, hop, run etc) This worked really well for us. Sometimes he wouldn't need to but it got him used to having to make time to go iykwim? I would make a huge deal out of it, high 5's, tell him he was a big boy like (daddy, a friend, puppa etc) I still have to ask him from time to time as he will forget but he does all his wees in the toilet now.
Good luck, i know it can be a very frustrating time!