It turned out our dd7 needed absolute control over her movement, wobbly training wheels or parents pushing freaked her out. We bought the ebook "riding made easy", and felt silly about it, but it worked. As others recommend, start by taking the pedals and training wheels off the bike and drop the seat low enough that he can easily reach the ground, and encourage him to take bigger and bigger steps. I won't give away all the author's tricks but that should be enough to find out if this makes him more comfortable.
You could get a wooden scooter bike instead but they are expensive, and just taking off the pedals makes the later transition to pedaling less stressful.
