This is kind of a long story, so I'll try not to go into unnecessary detail, but here goes;

There was a death in my family recently, and I had to fly out of my home state to attend the funeral with my mother. I am currently staying at my grandfather's house in a small country town. My grandfather's neighbor across the street owns a medium sized dog who is not kept indoors nor in a fenced yard, therefore, he is allowed to wander the streets freely. His fur is dirty and covered in burrs. My mother says he has a few ticks on him and I suspect from the way he kept scratching himself that he also has fleas. He has a collar, but no tags for identification, and also none to indicate that he has been vaccinated. He is somewhat thin, but didn't appear to be malnourished/emaciated. According to what my grandfather has witnessed and relayed to me, the dog is poorly treated by his owners, and he said to me that they "beat" him often.

The dog has a very sweet and loving temperament once you earn his trust (he cuddled with me for almost an hour today), and I feel that he deserves so much better. I worry that he could be hit by a car or taken to the pound since he is a free-roaming dog. I also worry that he is frequently exposed to the elements, i.e. rain, heat, etc. but since I have not witnessed these things myself and have only been in town for a few days, I don't know what, if anything, I should do about it.

Should I call animal services, a rescue group, a local vet? Should I let it go since I don't have any solid proof that they don't bring him inside, or whether or not they "beat" him or just punish him?

I want to help him but I don't want to cause a disturbance if it isn't absolutely necessary. I will only be in town for two more days also. What would you do?

I would definitely call someone. I am in Australia so I am not sure what your organisations do but here I could call someone and they can come and assess the situation and if they think the animal is being neglected they would take it away, potentially issue the owner with a fine and then rehabilitate and find the dog a new home. Maybe the rescue group? The ASPCA? I think that if you don't do anything you will regret it and always wonder if the dog is being maltreated. If they come and decide the situation is fine then you won't have to worry about the dog anymore. I mean they may love the dog and just don't understand about the fleas and keeping them inside and stuff but if they are genuinely abusing the dog then that is something else. I think the fact you posted on here suggests that you are worried enough that it is worth doing something about it. Maybe you could get the ticks off yourself with some tweezers? They are potentially fatal to dogs.

Thank you @ Mokat c: I've had several other people telling me it's worth reporting, so I'm going to take care of it today.

[edit] Smallish update, I reported the suspected crime anonymously so hopefully someone will check it out. I did find out that they have a fenced in backyard, but the dog gets out of it just about every day. I'm not sure if they're breaking any laws, but I really hope something can be done about it :(

My advice def would have been to call animal control, so good on you. Either the dog can be taken, treated and rehomed or the current owners will get a scare that'll encourage them to take care of their animal.

Absolutely call someone. And even if the authorities decide he isnt actually being abused, well then you still know you did the right thing just incase.