You could use a bike with a parent handle instead. That way he could still feel like he is a big boy and you could use it to teach him about listening and road rules at the same time. I do think it is important you teach him about stopping before the road and not leaving your side whether he is in a pram or walking.
DS is 3 and I still use a stroller for him when we go to the shops, somewhere busy or are having a long day out. He walks too slow and gets distracted easily and I need him to still be agreeable to going in the pram when I have our next baby in Jan (so I don't have to worry about losing him).