Hello Melissa,
This is what we did with our son Tallan who started playing guitar at 5.
He started playing a full sized guitar right away. He's arms were not long enough to reached the end of the neck, so we used a capo at the thrid fret. You can adjust where you set the capo depending on his size and reach.
We found that one problem was finding a music store to even consider giving him lessons at 5, most told us he would have to be atleast 7-8 before the would sign him up. If you have someone that will take time with him now that's GREAT, if not just keep talking with different stores and you'll find someone willing. You could even ask them for a trail lesson and ask them to just give you a 15 minute lesson. Most instuctors are worried that young kids don't have the attention span for 30 minutes. So a 15 minute trail lesson will atleast give you a chance to really see: first, if the teacher is right for you and second, if your son is ready for lessons.
We started Tallan with 30 minute lessons and it wasn't enough so we added different instructors during the week to speed thing along. Now he's taking lesson for an hour a couple of times a week. It will all depend on your son's passion to learn and most importantly his willingness to practice. Without pratcice, lessons are useless.
Another thing we found out is the 3/4 scale guitars don't like to stay in tune. When Tallan's playing his 3/4, he's constantly retuning.
Can't help you with the lefty thing but Fretwork is probably right. The guitar industry seems to favor right handed players. A lot more options.
IMHO, if you're afraid to take your son to try out some guitars because he might not leave, that means to me that he has a passion at a young age that most don't get til their teens.
Don't listen to anyone when they tell you he's too young, if I did Tallan wouldn't be where he is today. We are asked to sit in with bands all the time and he's only 7. It's never to young for music.
You can find out more about Tallan at his website: www.tallanlatz.com.
If you ever have any questions please let us know, we'll be happy to help.
Hope this helps and good luck.
Take Care,
Carl