If he doesn't do this, he may well regret it and feel guilty later on.
When MIL was terminally ill last year, dh spent as much time as possible with her - and it was very stressful, because she lived in the south of England, and we live near Glasgow, so he was away from home a lot - though I know it is not the same as having him away for weeks on end - but I am glad we managed to do it.
He was there, in the room, when she passed on, and I know it has really helped him deal with the grief. He got a sense of closure that has helped a lot.
However, he and his brother got a lot of help from the hospice that was caring for MIL - when she said she wanted to die at home, they sorted out a 24/7 carer, and support package (second carer visiting to help with personal care, and to give the full time carer a break, and district nurse visits to sort out her medication and analgesia) - and I think that was very important. If you and your dh decide he is going to do this, he needs to push to get as much support and practical help as possible.
All that said, what matters is that you support him in making the decision that is best for him, both now and in the long term. If he wants to care for his dad, you need to give him your wholehearted support - and vent/de-stress here. If you want to talk to someone who has been in a similar position, I am here, and more than willing to offer a shoulder or listening ear.
If he doesn't really want to care for his dad (maybe he's saying he does out of guilt or loyalty), then he needs to know that it is OK for him to say no to caring, and that you will support that choice. It would be perfectly understandable if he didn't feel able to take on this Herculean task.
I am so sorry that he and you are going through this - I have walked this path, and it is a hard one. Look after him, and look after yourself.