Some of the comment here verge on the ridiculous.
'Having access to the girls medical records?'
What you really mean is should the parent of the boy who is the father should or should not know if she is about to be a grandmother.
And the answer is Yes, because he has learning difficulties and the child, if it's born, may be looked after some of the time by the OP if he shares the care.
So of course she needs to know what's coming.
OP ignore all the posters having a go at you.
Both sets of parents - you and the girls- need to talk.
Your son and her are still children. They are not legally adults.
It would be very hard indeed for him to 'father' a child, when he is a child himself.
The odds of them staying together for life are very low indeed.
Your son needs to start understanding that whatever he feels now is likely to change.
I used to work with teenagers with learning difficulties. Some girls did get pregnant on purpose and wanted a child to 'love them' . They totally misunderstood the lifelong responsibility.
I hope you can talk some sense of reality into him and they might decide that a termination is something to consider.