My son plays viola and has outgrown his 14.5" instrument. We own it and it has a gorgeous sound. He wants to also learn the violin. Can we restring it with an E string? It's a half inch longer than a full size violin, so I just want to make sure it's safe. (I've had a guitar string break and it was terrifying. I definitely don't want to go there again!)
If you want to just try it out for a short time, and don't mind using up some strings, it could be worth attempting. This page says that a 14-inch viola (not 14.5) is pretty much the same length as a violin. What really matters is "vibrating length"—the distance between the nut and the bridge. If this is much more than the violin's 12.8 inches, you might have to get into different gauges of string.
But if his desire to play violin is certain, and is going to last longer than a quick trial period, I'd go ahead and sell the viola and get (or rent) a violin. As the page linked above points out, the viola is probably thicker than the violin and therefore heavier; its bow also is a bit heavier and stiffer. The heavier instrument can mean some fatigue, or alter your instrument hold a bit, and the different bow would affects some advanced bowing techniques like sautillé. Most of all, if the vibrating length is much different than violin, then the intonation will be off.
Ultimately, if he wants to continue playing both instruments, then he'll need to develop some differences in technique and switch between them, so the sooner he starts that process the better. (Or maybe he just needs a 5-string!)