I would not recommend it. Full size violins and violas have significantly different lengths, so you may end up in something with a similar length, but:
- viola strings are thicker and not as tensely stretched
- the instrument is optimized for different resonance frequencies.
So you would have to restring the instrument, risk damage due to higher tension and it is not guaranteed, that it sounds well afterwards.
Given, that viola players are typically in short supply in amateur orchestras (quite the opposite with violin players), your son could switch to viola, however. The teacher of our son regularly let her students try playing the viola - it's always possibly, that the student finds it more pleasing. The need to learn the alto clef is a disadvantage, however, even if some casual players use mental short cuts to map the score to treble key.