Mr. Speedy!

About 7 months ago I purchased a QH/Welsh gelding, he is a very quiet horse but after about a month or so he developed this speed problem. Of course I do love to trot/lope but sometimes it's nice to just walk. Wheither we are in the arena or on the trail he still likes to go fast, it get's frustrating. A couple days ago I was working in the arena with him and did lots of stops and used my seat alot, which made him seem to listen to the voice and seat better to stop, since he doesn't listen to the reins very good. I am wanting to get into barrel racing with him, he does have the speed for it, but how am I supposed to do that with him if I wouldn't be able to control him, right? What are some things that I can do with him to get him to listen to the reins and to me better when riding..and to just slow down?
Thanks!

We call Vida the Mosquito because she likes to buzz along We did lots of circles and half halts using the verbal cue "easy". She still gets quick once in awhile but she is much easier to keep at a steady pace now. Whenever you change speeds it has to be your idea not the horses. If you let him slow or quicken without a cue from you, your teaching him he doesn't have to listen to you for setting the pace.

That was some good advice, thank you.
I am probably letting him do a little trot or so whenever he wants and not even noticing it! Which developed into this.