My husband owns a landscape construction company. I see that you are from Illinois and your weather is usually somewhat similar to ours (I'm in Ohio). This spring, we had the rainiest months of April and May in recorded history. Usually, my husband will put in about 80-90 hrs/wk during those months. This year, he was lucky to put in a 25-30 hr week. That alone, should explain the delay on your work. With my husband's business, his regular customers typically schedule their spring clean-ups or new projects in Jan/Feb for Mar-May. For customers requesting work in May, their jobs usually begin at the end of June through the month of July. Landscapers get VERY backed up (especially the good ones!) and that is just a fact.
My husband has been in the business for about 15 years and most of the time, he is relatively accurate (w/in about 5-10% margin of error) in his estimates. On occasion, he runs into a situation where he has underestimated the cost of materials or labor. It IS possible. Your guy seems to have GROSSLY underestimated though. I have to be honest. A $700 job is barely worth the time/labor/effort to show up (which I'm sure has contributed to his procrastination on showing up). Sometimes my husband will pass on a job that small because after you pay labor and materials, there are virtually no profits.
Anyway, I doubt he "bait-and-switched" you with his estimate, but if you're uncomfortable, you should call him and tell him about your concerns. My husband is almost always willing to work with his customers and if he can't, he just cuts his losses and moves on. But beware, you might have trouble finding someone good to come out this late in the season in a timely fashion. You might end up waiting another few weeks- but this time you have a torn up yard.
Good luck!
PS I wouldn't bother reporting to the BBB. BBB is absolutely bogus. All a company has to do to bring their ratings back up is pay a nominal fee- then they're right back at a great rating. I know this from a couple of business owner friend's personal experiences.