I'd be happy for the low grade and tell him that he finally gets to experience what all the other kids do. And tell him the grade doesn't matter in elementary school so feel free to just learn. That's what I told my kids-- my dd got a few grades of b after her second skip and by the end of the year, she had straight As. Grades of A that meant something to her. And we discussed how much better it feels to get an A for working rather than just having them handed to her for showing up. It was a great teaching moment.
Yes, yes, yes! Hope it's not too hard for your new 4th grader and that you are able to reframe the grade in this way. An important shift.
Now in middle school, we just have "mom, I want to go through the quarter without losing any points". Really 100 average is kind of sad as an attainable goal. You only have a small window of time to teach these kids and I'd not cushion the blow at all. Use if for teaching purposes. And so he'll feel the unconditional love now in case he gets to perfectionistic later.
I was a kid who got 100+ all through school -- did you forget about extra credit, MON? (I know I didn't really consider it "good enough" unless I'd also aced the extra credit. *Groan.*) But that was simply what I expected. My grades never really meant anything to me until I got to college and had to work for them.
It's funny, even getting to go to Northwestern for some award ceremony in jr. high based on my SAT results or being Nat Merit never meant anything until I was reading parent posts on other threads here. They weren't a big deal, because I'd just shown up for the tests. Now, they served as reminders to me that I need to advocate for my children so they get to do meaningful work and be evaluated in ways that matter, ways that ask them to show up, engage and work at their own levels.
I hope this means you are getting that chance this year with your child and that you can help him realize that opportunity is worth way more than a certificate. (And, yes, no reason you can't celebrate the skip -- and the learning -- with your own award!)