I totally agree that the production of plastic is an awful burden on the planet, but to be honest - I dont really like wood that much either.
Rather than saying "plastic is bad, wood is good" we could think through each acquisition carefully. But how exhausting, really. The hardest for me is bucking the culture of consumption; just switching my desires from one kind of toy to another doesn't address that.
BTW, my son has acquired a trike and 3 bikes so far (sounds crazy I know, but he started riding a 2-wheeler w/out training wheels a few months after turning 3, and used several sizes, as both his height and skill level increased dramatically in the past 8 months that he's been riding). All handed down or super cheap at second-hand shops. We have now passed several along-- the tiniest 2-wheeler has become community property on our block, and has been the "learning bike" for at least 3 more kids that I know of. Bikes are one item that is very easy to come by used in ridable condition. (However, I do understand your earlier point and deal with the same issue, which wasn't limited to buying your son a bike).