Behavior or health problem?

We got our first horse (Cochise) two months ago. My teenage daughter & I both have moderate experience with horses (I owned one when growing up; she's had 3 years of lessons). We're far from experts, though. We got our 15-yo QH gelding from a horse trader who told us that the horse had spent this past summer at a kids camp. In our test rides, and for a while after we brought him home, Cochise was quite friendly and obedient. He did give the farrier some trouble on his first visit, though.

He's still our only horse. The closest equines are two doors down--close enough to yell to, but not close enough to really interact with. He generally spends 24x7 grazing in the 1.5-acre pasture adjacent to his barn stall.

My daughter (his primary caregiver) hasn't spent as much time with him as she'd expected to this fall. She'll try to groom him now & then, but her claims of "I'll ride him every day" have turned into us prodding her to ride once a week. The colder temps (Nebraska) and shorter days aren't helping any. We do not have an indoor arena, just pastures. Despite not getting as much hands-on time with us, his care has always been very friendly and loving. He's well fed and (unfortunately) never forced to work hard.

During the past two months, he's gotten progressively more ornery. He went from nuzzling us, to ignoring us, to yanking away, to now biting when we try to saddle him. My daughter claims that even a month ago, he reacted more poorly when you'd try to brush or pet his back or near the base of his neck.

That leads us to wonder whether his poor behavior is perhaps due to some pain in those areas. The pre-purchase vet check in another small town came back clean, except for somewhat flat feet. We haven't had our local vet look at him yet. We were hoping to make use of their Spring sales, but we can have them make a special trip if anyone thinks it's likely to be necessary.

I've been told many times that the cure for a misbehaving horse is a wet saddle blanket, and despite my prodding, that's something that my daughter has yet to give him in the two months we've had him. Last weekend, we were unable to saddle him at all due to his biting, which makes working him difficult. We do have a lunge line, but we're all pretty new to using it.

So to you experts: does Cochise's behavior sound more like a health problem or a behavioral problem? Is this just something that we need to work out of him, or is it worth paying for a special vet exam? I hate to work him harder if he's in pain, but I also hate to pay for unnecessary vet visits. Any advice would be appreciated.