Dog skinny but has food available?

Is he eating at all?

That's the issue with free feeding, you aren't able to monitor their eating habits and changes as closely.

Definitely speak with a vet, bring in a stool sample to test him for worms.

It really doesn't matter when was the last time he went to the vet, as he's losing condition now - and is no longer "fine". He is lacking the bloom of good health and getting skinny.

It could be a host of things - parasites (worms or heart worms), a tick borne disease, a problem utilizing his food, cancer of some sort. He could have some problem teeth bothering him and causing him to not want to eat, too. ... etc, etc.

I'd take the dog in for a wellness check to your vet because he's losing condition. Take a stool sample with you so that he can run a fecal test and see what types of parasites and other little nasties he may have.

The vet will most likely run a few simple blood tests to check for the basics. If your dog hasn't had a heart worm test this season, then that would be one of the first test to run. (Unless you keep the dog on preventive)

There has to be a reason behind why your dog is losing weight, and getting skinny - I hope your vet can help you figure it all out.

It's frustrating knowing that something is wrong with your dog and you don't know how to help them.

good point by rustupd, but as you mentioned the vet has seen your dog and he assures you everything is fine. if there has not been a drastic change in anything, specially his diet, maybe you can ask your vet if you could add a vitamin of some sort just so you feel comfortable about his weight loss. we always know our dogs the best, only you can know exactly when to be worried. maybe adding a vitamin so he can gain his weight loss back will make you more at ease.

Take him and a fresh fecal specimen to your vet and then start feeding him properly:

http://i40.photobucket.com/albums/e224/GLLNTKNIGHT...

worms ?