People on the autism spectrum are often 'perfectionists' and get furious with themselves if they get something wrong. (I know that only too wellm, because I'm on the autism spectrum myself).
For example, somebody might stop me and ask me for directions. Later in the day I'll realise that I sent them the long way round, adding a minute or two to their journey. I'll feel really bad about it for days and the experience will keep haunting me for decades. (For example, I still keep kicking myself about some poor directions I gave to a motorist in Sheffield 35 years ago, even though he will have still have reached his destination without needing to ask again. Even thinking about it now is making me tense up).
So, if 98% in a test is never good enough for your grandson, he might well be on the autism spectrum. (For me, getting 100% can be equally frustrating because I then start kicking myself that I can't do it every time).