Percussion choice was huge for both my boys - 4 years apart. Every kid wanted that as first choice. OK not every just many. My oldest picked percussion in 6th grade and got it. He stayed with it for 3 years. His teacher approached him and asked him to switch to French Horn. He and I were like huh? However my son liked the idea! This is his 4th year and he still loves it.
My youngest picked Trombone.He stayed with it for 3 years and decided he would like to be in chorus instead! Alrighty then!
I wouldnt put too much stock into what he starts out with. And - since he did pick it and it wasn't just assigned, let him try it! If he isn't happy he can change. Let him feel proud that he got what he picked and see how it goes.
The really frown apon changing once your instrument has been assigned so he will have to play it the whole year. I know he will do really well, its just that he has very low self esteem coupled with a very pessimistic attitude so at first when he doesn't get it, he will withdraw from it and it will become a huge fight to get him to continue. It goes deeper than that too, he will convince himself he isn't good and he never will be no matter what we tell him. I was hoping he would get percussion since he already knows how to play from his experience on dh's kit. I think it would have been great for his self esteem and we could avoid what is ineviatble (sp?) when he starts the violin.