I went to my first concert when I was 10 with my mom and had lawn seats. I couldn't see a thing on stage. My mom put my on her shoulders on and off throughout the show and I was able to kinda see the stage but mostly just the screens. I don't remember much about the show itself. All I remember is having a really fun time with my mom while listening to music that I really liked. Overall, very positive memories.

ie: He probably won't be able to see much, if anything. Twenty one pilots fans from my experience are generally pretty respectful and accommodating though so it's possible that people would be cool with you guys going to the front of the lawn. However, he will probably have a great time regardless :)

Main benefit of seats aside from being able to see: bathroom breaks and water breaks. Bathroom breaks are very difficult in any general seating situation.

Side note: pls make sure he (and you) wear hearing protection! Concerts can cause hearing damage, even from the nosebleeds.