Hi Cristina,
I started wearing glasses when I was a young baby (my mother thinks at around 15 months). My mother was also very worried about how to keep the glasses on me and how to avoid spending a fortune replacing broken ones. Turned out that once I put the glasses on I could SEE SO MUCH BETTER that I didn't want to take them off and I was very protective of them. She even had to let me fall asleep with them on and then sneak back in to get them at naps and nightime.
Recently I learned about Zenni Optical, an online store where glasses can cost as little as $8! (www.ZenniOptical.com) My husband ordered a pair recently and is very happy with them. Should you choose to buy glasses from Zenni or some other discount store I do suggest that you ask the optician at you pediatric opthamologist's office to double check them since your son is too young to tell you if there is a problem.
Good Luck!
E.