I started toiled training my little boy at 2 1/2, and it took forever. Many many wet jocks, poos on the floor, it was not fun! He would not sit on the toilet or even a potty, even if I put him on it every 10 mins, so he would hold on ALL DAY until he was so busting that he wee'd or poo'd his pants and then started to cry from embarrassment. I started using encouragements, like stickers or a few lolly snakes if he did manage to go in the loo and not in his pants. One day, all of a sudden at about age 3 (he was quite old) he just suddenly "got it" Do you let him watch his brother/dad/you go to the toilet? So that he sees its not a big deal? Maybe you could get him to watch your oldest go, and make sure you make a huge deal out of him, hes such a big boy going to the toilet, you are so proud of him, and give him a treat in front of your other child and emphasise that its for him going to the loo. (he may be upset initially that he doesnt get a treat, Be strong! but he will realize that to get the treat, he needs to use the loo, or to try) If he does a wee or poo in his pants, leave him wet for a minute so he "feels" the yucky feelings of wee's and poo's and that it is uncomfortable, before you change his pants. Also, in my experience those training pants were not much help, just like a nappy.
Does he go to childcare? Sometimes seeing his peers go can make a huge difference (in the last 6 months after seeing it at childcare, my little boy now stands up to wee) Also, something I do to improve my little boy's aim is to draw a target on a pingpong ball. I put it in the toilet, and he aims for it. Its made a big difference in spills on the bathroom floor
And it doesnt flush down either.
My son is 4 in a week, and pooing on the toilet has really only started in the last 3 months or so, mainly because I think now he really recognises that it is a yucky feeling. We are working on him not weeing the bed at the moment