T.S.
My son learned to ride on a wooden balance bike at 3. He got on a regular bike and rode alone on his first try about two months later. I'd highly recommend this over training wheels...
That said, in hindsight, I might have just gotten the 12 inch bike and taken the pedals off. If you do that (or when you're little guy is ready to transition) tighten up the front steering column so it's tight and doesn't turn easily... it's a lot easier to balance that way, and they learn to turn by leaning instead of steering dramatically (which is more the way an experienced rider rides, anyway).
You can get a balance bike for about $75. Prince Lionheart makes a good wooden one (that's what we ultimately bought. It has all the same features as the most expensive wooden bikes).