I'm sorry to hear about her death. It'll be painful for the both of you.
I'm wondering how R is going? Is he willingly doing everything you have required?
I'm concerned about the way you see discussing this with him. Why would reminding him of what he did last time and communicating your fears and triggers regarding the loss of his mother be like kicking a dog?
Does he make you feel like that when you bring things up or is this what you assume he will feel?
This is a part of life. This kind of situation will always come along. Something that tests their ability to prove things are different. Sometimes this is how BSs find out that nothing has changed.
Just because he is in pain, it doesn't dismiss his responsibility to continue making you feel safe. He doesn't get a pass because this is difficult for him. Now neither of his parents will pass again but there will be other stressors. He has to continue with the work of R while facing his own feelings. This should be a time where the two of you come together and work through this as a team.
It's not rude, selfish, or cruel to need additional reassurances from him during this time. This is the situation he created by choice. He should be falling over himself to prove to you, this time things will be different. This is a chance for him to earn back some trust.