My daughter used to do this when she was 5.  She wouldn't even take peanut butter on the bread the way she used to.  I would try everything to get her to eat something other than bread, but she insisted on just bread. Then something occurred to me: she only ate bread because I let her.  
 
If you allow them to eat only bread, they will eat only bread.  You have to put your foot down.  Kids get very hungry after playing, so it's not likely he's going to go on a hunger strike just because you insist he eat something without any bread on the plate.  And if he refuses to eat it, you leave it there and tell him it's all he gets.  Don't give in later and give him the bread because you fear he will starve.  It won't hurt him to miss a meal or two, and by giving in, you show him what your weakness is.  He'll start using hunger strikes to get anything.  
 
You needn't be afraid of his obsession with bread, either.  It's a "fun food" for young kids.  It's easy to snatch and gobble up, and it's fun to play with.  I'll bet he makes little balls out of it, right?  Peels the crust off and rolls the soft center in his hands before gobbling it up?  And it's within his reach, too.  That's why he likes it. It's a fast and easy snack, fun to play with, and works at filling his belly for a while.  
 
Place the bread where he can't get to it and in its place, put a ziploc bag of sliced or diced veggies, bowl of fruit salad, diced chicken with mixed veggies, etc.  Let him see other foods are quick and easy and fun, too.  Just remember, it will be easier to end his bread obsession if you stick to your guns and refuse to give in, even under threat of a hunger strike ;) .  
 
Oh, and this means no bread at all for a week- not even on his plate to go with dinner.  He'll eat the bread only and you're back to square one.  After about a week of no bread, he should be getting used to other foods again.  Letting him have a roll with dinner or a sandwich for lunch should be fine. If he tries the bread only thing again, start over again for the remainder of that week.  He'll realize he messed up and you were serious about not eating only bread.
 
I'm not a nutritionist but I was exactly where you are.  I did all the tips from doctors and nutritionists, which do come in handy now but didn't help break her of the habit.  So, this is just advice from a mom who was there.