The person you should be talking to is your son. Your daughter-in-law is touchy around you. She may feel criticized or guilty. Whatever the reason, she is not prepared to listen to you or respond to you. I suspect it makes things worse when you try. From her point of view, you are causing her more stress. To her credit, she is getting treatment.
Your son, on the other hand, has more standing with her. He loves her and is the father of the baby. If his wife is having difficulty, it is his job to step in and make the hard decisions. When in doubt, the care of the most vulnerable and unprotected comes first. In this case, it’s his son, then his wife, who needs his care and protection. If he can’t provide more practical and emotional support himself, then he needs to find the resources to ease the situation. Perhaps, for example, having a few hours a day of childcare would give your daughter-in-law a breather, so she could give the baby more attention when he is with her. If your son doesn’t share your concerns, it is his right to draw a boundary between you and his family.
However, if you feel your son’s judgment is clouded and neither parent will listen to you, you can call Children’s Protective Services and make a report. In most states, such reports are confidential. Talk to the case worker to see what the protocol is in your state and what help they may be able to offer the family.
I wish you well.