Yes, the new addition to the family could be affecting him greatly. It could be that it was very important to him to be able to play with the neighbor friend, and whether it was rejection (the friend didn't want to play with him) or it just wasn't a good time (dinner time) or for some reason you guys didn't want him playing with the neighbor friend (bad influence?) --- it could be he doesn't really understand what's going on and perceiving it that for some reason it's his fault he can't play with the neighbor friend. There could be a lot going on that he doesn't exactly understand.
A couple things came to mind...let him know it's okay to get angry sometimes, it's the way he deals with it that needs to be addressed. We have a 4 year old daughter that will tell us she's either sad or mad at us for various things - she doesn't want to go inside yet or go to bed yet, whatever. We tell her it's okay to be mad/sad, but it's not okay to have a meltdown about it. We'll tell her we appreciate her letting us know she's upset, and that we understand why she's upset - however, we still need to go inside so we can have dinner - or we still need to go inside and get ready for bed, whatever.
It's really hard to be patient with her sometimes when she gets so upset that we have to go inside or we can't do whatever it is she wants to do. But things do go so much easier when we try very hard to be patient, and get down on her level and say, "Okay...calm down and tell me why you're upset - let's use big girl words..." and try to get to the root of the problem. Depending on what the problem is, sometimes we can negotiate a little...if she wants to go to get "chicken and fries" at CHik-fil-a on Sunday, we have to tell her, "I'm sorry, sweetie - they're closed today. Maybe we can go tomorrow if you're really good, okay?"
We do make sure that she gets to help us with her sister...like "Hey, can you get the sippy cup (or nuk or whaever) off the table? Baby sister really needs it." And then give her a big "Thank you so much! That was a BIG HELP!!"
I also make sure everyday when we turn out the lights for night-night that I tell her good things she did that day, like "Hey, you did a good job today helping mommy with the laundry, and I really liked it when you helped me change your baby sister's diaper; you were a big help!" Her face always lights up when I do that.
Just a few thoughts...