How do I stop this behavior?

My first suggestion is keep him on leash for his safety. If he takes off while he is off leash, he can get hurt or killed and I'm sure you don't want that to happen. Secondly, this needs to be proofed, challened. However, the trick with proofing is that 1) you have to be as reinforcing if not more reinforcing than the environment. and 2) you need to be able to stop and prevent inappropriate behaviors from occurring and becoming habit. So, a long line can be used. There are lots and lots of trainig methods and techniques that go along with what you want to do. My suggestion is to find a positive trainer in your area.

Best answer is to keep him on the leash. Dogs should not be off leash outside a fenced in yard, you never know what they will see and run into the street to chase. Most cities have leash laws anyway.

My dog will not leave my side for anyone or anything but I still don't let him off leash just in case. It's just not safe. My suggestion - don't let him off leash unless you're in a fenced in area.

Congratulations! If you can keep your dog from wanting to chase a squirrel while on leash, you are halfway there! WOW! Now, just work with him in a safe enclosed, e.g. fenced in area with similar but lesser distractions.

In a confined area like someones yard, I would first see if maybe the lack of the leash makes your dog thing that he can run wild. Assuming thats correct, you need to train to have him walk by your side WITHOUT the leash. This should do it.