First, I'm so sorry about the surprising news that DH needs a pacemaker. I'm inclined to agree w/ other posters, though, that this is probably a precautionary measure, if that's any comfort.
I understand your expecting a greater show of concern from DS and DIL. I'm going to chime in w/ others, however, and say that they probably don't see a pacemaker as a worrisome procedure and don't realize how anxious you two are about it.
As for DIL, I'm another one who's wondering why you seem more upset w/ her than w/ DS (am I reading that wrong?). I was thinking, as Agnurse, said, that she and DS may have a yours/mine policy, so she is leaving it up to him whether to contact DH or not. Or she may just be following his lead, if he doesn't think it's serious enough to show concern, she's not going to either, Or as others have suggested, she may just not want to miss a minute w/ her own mum .... just some possibilities...
Hope all goes well for DH! And that if you let DS know how thrown you and DH were by this new medical situation, he'll show a little sympathy and maybe come for a visit.