Sounds like he sees using the potty not as a positive experience, but merely an obligation that does not get him any of what he wants – and you are right, if you have a baby in the family (for how long, now?), he wants more of the attention going to the baby. Because of that, regression is so common as to be "normal" after the arrival of a baby.
You say he was getting "close" before the baby, but that could mean any number of things to you that might have a very different color if you could ask your son. What was his age at the time? If he was three or younger, that's really pretty much on the early side for a boy to be successfully trained. Many kids, especially boys, often simply need more physical and emotional maturity for potty training to work. Some don't get there until around 4 or even later, and that's without the complication of major family changes.
Here's an extremely informative website you might find helpful. It gives a few variations on"readiness" checklists, plus tips on various training strategies, the best ages to start them, and the advantages and disadvantages of each approach: http://www.parentingscience.com/toilet-training-readiness...
As difficult as it is to have two children in diapers, long, difficult training with many accidents is probably even worse – certain worse for the child. There are so many ways negative attitudes and fear of failure can creep in. So you might do yourself and your son a favor to put off further training for a bit, until he can come at it fresh and perhaps with a greater ability to connect the dots between urges and 'outcomes.'
There are also lots of positive things you can do: continue to make all your messages about using the bathroom as positive as possible. That can include modeling how easy/quick it is for you or Daddy, reading potty books or watching potty videos, having his toys/stuffed animals role-play pottying, and in general making a game of it. With my grandson, once he could do it but didn't want to take the time, I would go into the bathroom and begin noisily ejecting all the dinosaurs that were crowding the room and sitting all over the toilet seat. My grandson couldn't resist that game, and would come in and help me wrestle the beasts, and claim his spot on the toilet.
Wishing you the best.