our dogs bum is bleeding!

Hi all I am new and have signed on to this site with a couple concerns about our dog barnum. He is bleeding from his rear end recently and it is darker than last time (last time was about two weeks ago or so). The amount is also much more.

One of the concerns i have is whether or not he gets to many milkbones and if that could cause this.

Another concern is that he is getting old and this is age related.

Last concern being that it was something he ate ie. glass, metal or something like that.

He is not my dog but my step mothers. I am not less concerned due to this but he has not been brought to the vet to be looked at yet.
Hoping someone here may know something based on that.

P.S. He used to be allowed to eat alot of vegetables. carrots, broccoli, and some lettuce