I think that as long as your DS isn't putting things in his mouth anymore you're probably ok. Our DS is 19 mo now and has had a few hot wheels type cars for several months, given to him by the older neighbor boy. He does great with them and loves them. That being said, I really want to start collecting the Plan City little wood cars for him. I'd rather he had just a few quality cars than an explosion of a thousand Matchbox cars, lol. I also found a great chemical free road play rug that we'll get for his birthday.
When he turned 1 we got him a couple of these cars:http://www.etsy.com/view_transaction...on_id=22282802
They still get played with pretty frequently. I'm thinking about getting a few more colors for him.