when you try to grab his collar to take it off how do you do it? do you go under near the chest or over the head? over the head in dog speak is a dominant position. when a dog "hooks" (puts his paw, head, or body) over the head or shoulders of another dog he is establishing his place in the pack. for this reason many dogs are uncomfortable when you approach over the head. you might try under the neck and if that riles him too then you need to recondition. a previous post mentioned training the dog that only good things happen when i grab your collar -- excellent approach! have him sit -- treat. touch his collar (from under) treat treat treat. repeat until he tolerates the touch. then touch it and move it - more treats. eventually you should be able to take it off with little problem!