Your mom should be encouraged to pick up the phone and call your nephew. She is the elder and if theres a problem with the relationship for whatever reason, she should be the one to offer the olive branch, call him and talk to him. IMHO.
Both my mum and dad have done this, they called him on previous eid and then once after that but the convos last like 2-3 minutes as he just stays quiet on the other end. When my parents discussed with his parents (dads are brothers) they were like oh he will come round its just a phase blah blah.
My mum calls his sister atleast once a week but she hardly ever calls my mum and never speaks to my dad.