Hi B.,
I know you already have lots of answers to your question, but I was quite surprised at them, so just wanted to give the other side. I am blessed to live about 20 minutes away from both my mom (age 62) and my mother-in-law (age 69) and equally blessed that they help take care of my children, ages 3 and 4 months.
They each have one day when they come and help me in whatever way they can. When it was just my 3 year old, they came when I had to work (part-time, which I am no longer doing) and now with our baby, they usually come to take care of my eldest while I tend to the baby. Once in a while, they watch both kids, but only for a few hours. I know they are sacrificing their time to do this and it can be hard to carve out this time every week year after year, but I also know that they love the time they spend with their grandchildren. If they ever have to miss a day, they always find another day when they can come to make up for it.
I think they are many things you need to take into consideration in your situation, if you feel well enough to do it that your health is good, and if your son is approaching the subject in a respectful and appreciative manner, for example.
As you said in your update, you're willing to give it a try, which I think is great. Maybe like someone else suggested you can do one day and he can find someone else for the other? I really hope it works out and that you end up loving the time you spend with them. There are lots of grandparents who watch their grandchildren full-time and love it and others who would rather not have much involvement. Maybe you just need to see what fits you. As someone who has benefited so much from grandparents' help and who sees just how much my children LOVE their grandparents, I hope that it will work out for you, too.
Take care and good luck!