She would feel I was undermining her life because she's never had a paying job. A soon as she got out school she got pg. So her life has been Troy and by asking to take him for a weekend would be crushing to her.
Errm, that's a bloody BIG assumption

TBH, I think its best you stay out of it. Support DH totally in HIS quest to make a relationship with his son, that's enough.
It's clear the way you feel about her, and that's another reason for you to stay out of it - otherwise it will be about you and her, not a father and his son.