Since he is still adjusting to the new sibling, I would suggest the whole potty idea to him as something that he GETS to start doing, now that he is getting bigger. Don't force him to use it if he is uninterested, but show him the potty and let him know that "now that he is a big brother, he can start using the potty if he would like to". Then show him how it works by letting him drop in a cheerio or something and let him flush it away. Explain how his body waste goes into the water the same way and then he'll get to flush it away too! If he is bored with the whole thing, don't push. But the next time you "happen" to be at Walmart or whatever, ask him to help you choose a potty seat (pick two and ask him which one he would like to use). Then when you are back home, be all excited and ask him to come help you put it on the potty. (he can take it out of the bag and throw away the plastic wrap, etc). Right then is when I would ask him if he would like to try using it. If he says no, that's okay. And tell him so. "That's okay, maybe you can try it later this afternoon" . Have a ziploc bag of candies (individually wrapped lifesavers worked for us) in the kitchen. When he is successful going to the potty tell him "that's great! I have something for you in the kitchen... wash your hands and we'll get it"... then let him CHOOSE which piece he wants. Each time he uses the potty successfully, he gets to choose a candy (even if he goes 17 times the first few days). By the time he is fully trained, he will have forgotten all about the candy, so don't worry about that.
I started trying to train my son when he was 2. That was a little frustrating for us, b/c he wasn't very interested yet. But by 2 1/2 he was and he was fully trained by the time his little sister was born 2 weeks before his 3rd birthday. Getting a small stepstool he could use helped (he never liked potty chairs, he wanted to use the big one, with a child seat). We continued to use pull-ups at night only for the whole time he was 3 though, b/c he slept so hard he would have accidents at night (still does sometimes at age 9).
Once he is consistent with peeing (which will happen before the pooping), take him on another trip to the store and find some cloth training pants for him. They are SO much more comfortable... let him know that when he starts going poopy in the potty too, then he can start wearing them all the time! You can even let him try them on (make sure you have the right size) so he can see how much better they feel than diapers/pull-ups.
I have heard all kinds of horror stories about boys and potty training. But I just don't agree that it is always that way. Don't psych yourself out thinking it will be awful.
Just stay positive and encourage him with lots of cheers when he tries and when he is successful. And keep quiet when he is not... (an "Aww, well, let's clean up. You can try again next time" is all that needs to be said). No muttering under your breath or heavy sighing... he'll read right through that and the potty will become a bad experience for him. Stay positive and he will want to please you. Start giving "bad" vibes and it will become a power struggle. Remember, it can take a month or two (or maybe even 3). As long as he is trying to get to the potty on time... there is a way to tell him "good job!"...
Good luck!
I have the cutest picture of my little boy sitting on the potty in his PJ shirt (no bottoms) and reading a book about using the potty!
You may want to keep some small books (that he can hold easily by himself) in the bathroom, so that he can distract himself when going poopy. Only let him look at those particular books when on the potty.
pps.
Just a tip -- since we girls don't know how boys learned.= and people don't always talk about this...
Don't worry about him standing up. He will sit for a while. Then, when he is doing both well, he will learn to stand up--- that part comes AFTER he masters getting there and using it successfully... lol. And BEWARD the FALLING TOILET LID... make him use a step-stool when he gets to the standing up stage. I am NOT the only mother who's made a mad-dash to the ER b/c of the falling lid "biting" my little boys' parts.