As Despos said, start working on your back-up plan. If you are not satisfied with the store tailor's (3rd?) attempt to make this right and the store is still unwilling to do right by you, you give the store their options. Tell the store they can (i) give you a new unaltered tux in your size that you will take to another tailor; or (ii) they can lose a sale and eat the cost of the botched tux because you will walk out and have your credit card company reverse the charges. For the second option, tell them you have already located a replacement that is on hold and will move on immediately if they do not make it right. The problem is that, with the store manager gone, the staff is scared to make any tough decisions or agree to eat any costs. You should calmly help them realize the shitstorm they will be in when the boss returns if their inactivity causes the store to have to eat the cost of a botched tux and deal with the negative publicity that you will ensure they receive for screwing up your wedding.
In the meantime, you should be working on lining up a replacement tux and a good tailor who agrees to turn the alterations around in time for your wedding.
Really good advice. Thank you!