DS3 was born 4 weeks ago... DH and FIL have quite a strained relationship as DH does not like his step mother, and despite them living 40 minutes away, we usually only see them once a year(ish).
DH phoned FIL to tell him that DS3 had been born, to which FIL said congratulations, they were about to go on holiday for 10 days and would call when they got back. And we have heard nothing since. 
AIBU to not understand why a grandparent would not bother to visit their own Grandchild? He has other "step" grandchildren from his second marriage, who he seems to now consider his real family.
DH is ambivalent about the situation, saying it's his loss if he doesn't want to bother with his new grandson, and won't call him. I feel I should call FIL but am worried I might say something inappropriate to the miserable git. So... should I stay out of it or give FIL a call and ask whether he is intending to visit DS3? 
DS3 is such a sweet baby boy, and the situation makes me feel like he has been rejected... I am seriously mad about it. 