This situation calls for you and your adult son - your grandson's father - to access your county's resources for daycare. There are programs that help pay daycare center's fees for your child's daycare.

Help Paying for Child Care | Division of Child Care Services | OCFS

I would also create a calendar that you can show your adult son, of your availability. And, if he doesn't like that, you can also suggest other people to watch his son the days you're not available.

If he berates, mocks, shames, gaslights you, become a broken record and just repeat your boundary to him, "This is the solution I have to offer." Something like that. Don't emotionally respond b/c you want to maintain your personal power with your emotionally reactive son/children. Just repeat your boundary and stay calm. Then, release your emotions when they are gone; cry, scream, laugh, swear...take a bath, go for a walk, listen to music and dance, call a crisis line and vent...to get your emotional needs met and your physical needs released from the stress so that you don't carry it around inside of you.