I would say it's alot to do with the new baby. Marisa was 2.5 years when Elijah came along and she went backwards in a major way - we had to put her back in nappies for some time as she would just act like a baby - she would just wee on herself and it became very frustrating. They see the things the baby does and it gets attention for, e.g. crying and it gets picked up - so they can revert back to baby behaviour too. Perhaps you can take some time out to have one on one time with your son as well as dad, so he knows he is still special and the baby which has interrupted his world isn't the only thing that he thinks matters.

You might like to read this article by Pinky McKay too: https://www.bellybelly.com.au/baby/a-new-baby it's called, A New Baby - Second or Third or More Time Around. Very interesting.

Good luck!