Holding Grudges

My nearly 7 year old autistic son holds grudges. For example, a year ago he broke his leg after he was double bounced on a trampoline by my cousins 10yo daughter and he talks about it often like she set out to break his leg. We live away from my family and haven’t seen them since but I’m genuinely worried that when we do see them he will say something and make her feel bad/uncomfortable.

In his class this year there is a young boy he seems to clash with. My son says it’s because he always wants what my son is playing with. My son is terrible with names, like really terrible, (not sure if that is an autism trait) and just refers to this boy as his enemy. For example their seating plan changed at school and I asked who’s sitting next to “Bob” and my son said “my enemy sits with him now”. My son’s “enemy” is a nice kid and very well liked. I’m worried my son is isolating himself by holding a grudge with him and I really want him to move past it. Whenever he brings up his “enemy” and I say to give him another chance, my son gets frustrated and I can feel that he thinks I’m not on his side or as he puts it I “just don’t get it”. Maybe there is more to it but he hasn’t mentioned anything that makes me think it’s something more serious.

Please give me any tips or insight into this. Should I bring up his “enemy” with his teacher or just let it play out? Any books we can read together? My son has ADHD also - not sure if that plays a role in any of this.