We have 15 training pants, 8 rubber pants to go over them when leaving the house, and go through them in 3 days on the average. I'm using the remainder of his Extra Protection Pampers for night (I bought BIG boxes from Amazon and want to use them up. Once they're gone it's Potty Scotty Night Trainers.) We're not using pull-ups because they whick the moisture away from the body (that's their purpose, to keep them dry) so I've found little ones have no incentive to pull them down and go potty and use them as diapers, plus, they're too expensive.
And anyone now saying I'm loony regarding pull-ups should take a look at the new Huggies slip-on diapers, same concept with a different marketing pitch, and still more expensive than regular diapers ; )
http://www.huggies.com/en-US/products/diapers/littlemover... *However, PT moms take note ~ one of the moms whose review I read said these work better at night than pull-ups as they absorb more.
My suggestion for what you need is lots of patience and a sense of humor. As Krista P said, your son still may not be quite ready, even though it's great that he's excited about his potty, loves peeing in the tub, and squats to poop. My guy was the same, and yet we are now preferring not to pee in the potty and are in major poop retention, Day 5. And he agreed that we should "throw the potty away!" (I recklessly asked this jokingly this morning because he wouldn't even sit to try and pee, sigh.)
We've spent lots of time with no bottom clothing, although he prefers none at all, to facilitate ease in training. But like any parent who's been there will tell you, it HAS to be their idea and if anything rocks their thinking all bets are off! I honestly thought we'd grasp PT 101 in a week or 2 tops, with accidents of course, but that we'd have the idea down. Instead he enjoyed it about 4 or 5 days then "NO!" was his every response. We took a break of a few weeks when stickers, Dum Dum pops and books didn't inspire. Then he started taking off his diaper again and saying "potty!" so we began again and all seemed cool. He was peeing practically on demand (no, I didn't) for a couple of weeks, pooped a few times in it and once on the carpet and was so proud :-/ Then we're back to "NO, Mama!" I got a few new ideas last night on training defiant toddlers, with him I believe privacy is an issue, and plan to implement them but if nothing works will back off awhile again, then restart anew. He's 28 months, won't be going to preschool until Sept 2012, so I know we have plenty of time.
Oh, and the humor part? Just in case you want to keep your sanity ; )
Hope all goes easy for you and your son!