Rereading the OP, this jumped out at me:
"Arrangements are often cancelled at the last minute with an alternative that is then not followed up /forgotten."
I can forgive infrequent contact (people are busy, maybe not as interested in contact as often as I am, etc.). But frequently canceling appointments and ignoring the alternative seems very rude to me, But it also gives me pause. I would be wondering if our relationship were strained for some reason. Are there any unresolved issues between you/his father and DS? If so, he may have put them aside for a while, after GD was born, but that doesn't mean he has forgotten them.
"The thing that worries me most is he has often become distant like this when he has problems in a relationship particularly with money and his girlfriend at the time. "
I totally understand your worrying about this. I would, too. Above, I said it was all the more reason to let him know you're there for him. But at the same time, it's all the more reason to give him space and be patient.
Please realize it could just be that he needs time to adjust to his new role. It would be great (maybe) if he told you this, but he doesn't sound the type to confide in his parents. Again, please be patient. Also, I suggest focusing on other aspects of your life (friends, hobbies, etc.)