Working with a green horse?

I have a quarter horse gelding. He is a little older than 10 and I was the first one to do anything with him. He strives to learn and does so quickly, but he gets easily bored with just doing groundwork and walking in circles or around poles. I have had him for three years and I have been riding him. He does a great job especially when on the trails. I have not started changing gaits yet, though. A few times we got into a trot, but i did not cue him into it, and he got skittish when I asked him to walk again. So I would just like to know how exactly I should introduce him to trotting/loping/galloping with a rider on his back. Thanks