My son was 15 months when my daughter was born (now 4.5 months). I get him involved in almost everything. If she was crying I would encourage him to gently ‘patpat’ her on her leg and sing/ shh her If I was changing her nappy I would ask him to bring me wipes and he would take the nappy to the bin etc Now 4 months later if she cries he says ‘uh-oh sister crying’ and then pats/strokes her xx
my daughter was the same way, i j started making it silly to her like making silly faces and he like ‘oh myyy he’s being so loud rn’ idk it seemed to help cuz she doesn’t bat an eye now
My nephew then 6 was the exact same way when my daughter was born and cried. We had to tell him she cries because she needs something and we have to figure out what is wrong. It turned into a game every time she cried and he loved to figure out what was wrong.
It's all new to him, and if he's not used to being around her or babies for a full day it will take some adjustment. You'll just have to reassure him that she is OK and it's the only way she knows how to 'talk'