Yes, this is new sibling 101. He's craving your attention and as another mom said, to him, bad attention is better than no attention. Consider this plan, in order...
Specifically regarding the "accidents", I agree with the mom that said put him back in pull ups for now and show him that he can still go on the potty and just use these as underpants. I know it seems counterintuitive to "go backwards", but if you can put that out of your mind, it will save your sanity. Take it off the table...make it a non-issue for now. He won't go to college in diapers. He has all the information and training on how to go on the potty. He'll decide when the final deal is done. Until then, it's like banging your head against a wall. I bet it won't take a week until he says it's time.
It's VERY important to spend extra time (yes, I know you have no "extra" time!!) with him ALONE. Give him 100 hugs a day. Remember, he's been the center of your universe for so long...he's sad to not have you at his beck and call all the time.
Next, (you're probably already doing this)try to find a few ways to consistently involve him as "mommy's helper". He needs to learn that the baby can't do ANYTHING for himself, but that the big boy can. He can be proud of all the things he can do "by himself" (or with a little behind the scenes help from mom and dad) and all the things he can do to help the family.
Try to praise him way more than usual...way more. Things like "wow, you're really trying hard!", "I didn't know you could do that by yourself, way to go!", "I love it when you help me by bringing a diaper to me!" That will boost his ego and he'll want more of the "good" attention. Try to ignore his bad behavior (that is, don't make a big deal of it...say it once then move on.) If you take away the reaction and interaction that's involved with the bad behavior, he'll find it not very interesting and not worth his while.
As he puffs up and sees how important his new job is and how important he still is in your family, he'll want to get back to the bigboy pants asap.
Good luck and God Bless!