I've known this guy for about two years and we always got along great. I know he was not over his ex girlfriend when I first started seeing him so we stopped talking for awhile. I was away for a couple of months but now that I am back I decided to give him a call and see what he was up to. I went over his house a couple days ago and he brought up how he wanted to go horseback riding with me like we had planned to do last summer when we were seeing each other... he picked a day and everything and seemed really excited to go but today he called me and said that he had to go to work instead... I know he really does need the money as he is in between jobs but I feel like this is exactly what he was doing to me last summer (making and breaking plans) so I'm not sure if he's serious or not and I don't really think that he is thinking that I want something serious... how do I let him know without being too obvious that I would really like to spend more time with him?

Spend time with him, and enjoy it. When you enjoy spending time with someone it shows on your face.

Your question "is he serious?" has a fairly obvious answer... no, he's not, not at all, not yet.

As to what you do to let him know you may be... that's pretty straightforward, too. You show up and talk to him in person and tell him, calmly and with no unnecessary pressure. He just needs to have the information.

He doesn't need to feel about you the same way at all, he just needs to know.

My wife turned me down the first half-dozen times I asked her out. Thus, she knew I was interested. I was persistent without being creepy and I didn't let her standoffishness get me down. It wasn't personal. Getting sad about it would've been pointless and helped ZERO. So I didn't.

Eventually we started going out... and the rest is roller coaster history!

No magic here, it's about sweet persistence and calm communication. You can do it.