I am quite new in Boxer / Dog World and as far as I know there are different opinions.
Fredi is now 5 months old and I have started using Orijen, which is grain free and high protein. Fredi seems OK at it and loves it. But I will need at least a month to make some judgment.
I read their FAQ and Whitepaper (both available on their website) and their concept sounds logical to me.
I think there is also Timberwolf Ocean Blue which is grain free and has lower level of protein if that is your concern.
Hope this helps,
velimir