Unfortunately anything you say will just antagonise him and play into his girlfriend's hands. Let him know that she's told you and say you're also trying for a baby so you will both know the joy of new fatherhood and he can have that special bond with his grandchild.
Keep in contact, even closer than before now the children will be good playmates. It will also royally piss his girlfriend off.
Infusion-delusion t1_j6jcclu wrote
Reply to My father (M58) is planning on having a child with his girlfriend (F33), who is 6 years older than me (M27). by ProfessionalManner25
Unfortunately anything you say will just antagonise him and play into his girlfriend's hands. Let him know that she's told you and say you're also trying for a baby so you will both know the joy of new fatherhood and he can have that special bond with his grandchild.
Keep in contact, even closer than before now the children will be good playmates. It will also royally piss his girlfriend off.