There are 4 total. (sorry for the lack of photo quality) The 4th one is down his rump a little bit more. They just feel like hard skin bumps, they do not bother him to the touch. I have never seen anything like them and considering they go down his spine I am curious... 

I bought this horse sight unseen a couple of weeks ago and this wasn't disclosed... The more I look at them the more I want to know why!

Thank you =)

Nope, but following!

Is he itchy in that area of the bumps?

THey look like " Warbles" to me. Seen them in cattle. Its a worm that works its way out. They eat the eggs, then bump forms, you can actually " pop" the worm out. If thats what they are. You will see a crusty top form on them first, then a small hole will form, then you can pop them. Have heard NOT to deworm because they will die inside???? Please let us know what you find out. 

I have one that has 1 bump that a horse bit him.  It doesnt bother him, but when the hair is raised it means he is out and needs to see osteopathic.  Its a great sign for me lol  it feel liks a tiny scar tho.  but he just has one and it is towards the loin area. but on top of the spine

No itch

You know I have seen these to, and a friends horse did have two pop up on her. I just could not come up with a name for these since I have not seen this very offten. Thats why I was asking the OP if the horse was itcy in those spots, my friends horse was very itcy. 

I don't think so. These are very hard and have no crust or anything like that. They seem to be more of a scar tissue type thing but since they are all down the spine I found it odd and it made me think. I haven't contacted the previous owner about them yet but I can message and see what he says.

Edit** Previous owner said he has had them since he has owned him and the previous owner didn't mention them. Said two vets have checked them and said they are nothing and two of the bumps have already gone away. 

So odd to me still!