Unless you sign a contract, you're not bound to anything.
Mortgage brokers are middlemen anyway. They usually get paid base salary + commission for a year and switch to an all commission salary after a year or two. If you were able to get a better deal through someone else on the first request, I say drop the first guy anyway. I don't like playing footsie when it comes to money.
I suggest shopping around on your own. Sometimes the brokers work with bad companies that will buy and sell your mortgage to just anyone. The reason I don't like this is because they can sometimes hit you with unexpected fees, depending on how honest the companies are that end up with your mortgage. Do research and definitely try to lock in a low rate this week or next...they're expected to continue rising throughout the Summer and into the Fall.