2-year-olds have pretty primitive emotional responses. He's acting like a baby because the baby is the one getting the attention, and he figures, if babyish behavior is what is getting attention, I can do that.
Give him as much alone time with you as you can. Don't ask him to be involved with the baby, the baby is (presently) a usurper and a rival to him. Just reassure him of your love, independently of the presence of the child.
He just feels that since you have a new baby you don't love him anymore he feels you only have time fire the baby what really helped me with my son was when my daughter was asleep I would sit with him and hug him and tell him how much I loved him and I would also play with him and I really didn't make things he didn't want to then he started to help with my baby girl on his own the misbehavior he's having is just Jim asking for attention even if its bad attention
My nephew did this when his 2 year younger sister arrived my mom helped the situation by explaining to him that he is a big boy now and has to protect his baby sister. My sister and mom gave him things to do to make him feel important. His older sister by 5 years also helped a lot by spending time with him. I know it'll be hard with a newborn but the older kids need indivisual time with mommy and daddy too. So maybe he can take the baby for a few hours so you can do something nice with the older kids.