Get a dremel with a grinding stone and shorten the lower beak a little. Go slow and keep it symmetric. There are blood vessels in the beak so go a little at a time. The dremel will often cauterize while you go to prevent bleeding.
I have had this issue with a cross beak, and I fed him out of a coffee cup, something deep enough he could push his beak into to get some, same with water. Rotten brat would stand on his coffee cup and squak at me when it was feeding time!!