C.M.
It sounds like your son is adjusting to having to share his mom and dad!! Even though he used to go to sleep fine, he's seeing more attention on the baby, so now he wants that more attention. My suggestion would be to have one of you sit in his room - tell him that you will not stay in there until he's asleep, but will stay for a certain amount of time (whatever you are comfortable with (5, 10, 20 mins) then tell him you are leaving but he has to stay in his bed. If he gets up and comes out of his room, walk him back. He may cry or get upset, but as long as you are giving him the attention before bed, he'll get over it quickly (hopefully!). When he gets up in the middle of the night, walk him back to his room (or go in there if he's just yelling at you) and sit for a few minutes, but do the same thing and tell him when you are leaving.
I say this but my situation was different from yours and we did things otherwise. My son was in our room from 0-6 months while my daughter (2 - 2.5 during those 6 months) would sleep in her room. Before he was born, she would cosleep with us most nights (wake up in the middle of the night and I just put her in bed with us). After he was born, she did not want to sleep in our room!! We used to have to sit in her room until she was alseep or mostly asleep then would sneak out. When we decided to move my son to her room to share, I knew that would have to stop so I started sitting with her and telling her when I was leaving. I also started leaving her door open instead of closed. The first few nights of this was a little hard, but it only took 1 night for her to realize she had to stay in her room. And the 2nd and 3rd nights we hardly got a fuss. Now I sit in their room and rock him until he's comfortable but not asleep and then I offer to rock/cuddle with her (she usually wants a hug and kiss then wants to go back into her bed). Then I say her prayers with her, sing her songs and then sit in there for anywhere from 5-10 minutes (depending on how tired I am!). This has worked for us, however, she still wakes up 1x every night and now that her door is open, she just comes into our room and gets into bed with me! It works fine for us because my husband doesn't get home until 230 am so he doesn't come to bed until later so we are not all squashed together!
Good luck! And PS, I would NOT try to sleep train your baby, he's just too young. Much better to sleep train your older son.