After weeks of lurking on Craigslist and some false starts, I finally managed to buy Ads a bike. It's a 12 inch Huffy bike with training wheels and he rides it very well. It's second-hand of course (I was determined not to buy a new one -- they are exorbitantly priced for something that will only be used for a year at the most) but in near-perfect condition. And I paid only 20 $$ for it!! That's the part that thrills me the most :)
He already has knee and elbow pads that came with a pair of skates (which, incidentally, he refuses to use) but I still have to get him a helmet. He's very pleased with his new "toy" and has thus totally ignored the scooter that is also a recent acquisition. The fact that the scooter is much harder to maneuver may largely account for his indifference. Ads likes things easy.