You may not have to buy it again. If the small lasts him for two years, you would probably be OK using a booster when travelling at that point (5 years old).
I would practice with the vest before you go to see how it works. It just takes some practice to make sure it is on and positioned correctly. Make sure that he sits all the way back against the seat when you are threading everything through so the lapbelt is down low. Good luck and post back with any questions!
Mama to three boys ('03, '05, '07)