So he's empathizing with the hypothetical reaction of his mother to his own death.
Yes.
Does that mean he is also grieving over himself?
In a way, Yes. It is entirely possible to grieve over fictitious scenarios if they are perceived (even temporarily) as real. In effect he is taking on the emotions of his mother (grief for him) and experiencing them subjectively. That is Affective Empathy.
or can he feel her general sadness without feeling her grief?
Yes to both. It basically comes down to how he perceives the event. To an individual subjective perception is reality, regardless of whether or not is based on the actual true reality. To that end what he believes she feels is in his reality what she feels. This is how we make all of our judgements in life, based on our perception of reality.
since it'd be hard to grieve over one's self?
It isn't hard to grieve over yourself, if you're experiencing that perception through someone elses perspective. Just like when you get really caught up in an actors plot on TV, you experience reality through their perspective. That too is affective empathy.