The parental obligations must be hard for anyone. They can be very stressfull. Bipolar people learn how to avoid stress. Not taking meds and combining this with alcohol, makes it absolutely easier, to get maniacal or depressed.
I never did drugs or alcohol, so that made my Bipolar Disorder a whole less complicated. I did ruin lots of friendships and relationships when I was still searching for the right medication and looking for the right therapy.
Being Bipolar made me understand much later in life, that I hurt and damaged people I loved, in a horrible and effective way. I hope things get better over time for your son and his children. I hope the mother of his children, finds her way back to good medication and effective therapy.
My psychiatrist told me that my first child, would risk 25% of inheriting my Bipolar Disorder. Between the age of 20 and 30 he/she, could develop the first real episode.
After what I've been through, not even 5 % would be worth taking that risk. Being a stable and responsible girlfriend is more than enough for me:-)