Labrador RETRIEVER won't fetch?

So I rescued this 7 month old lab from the streets about a month ago. He is the sweetest dog and very friendly and just loves people, so he's working out at a pet except one problem: he won't fetch. When I first got him he would unenthusiastically fetch for a little, but now he is a little clingy and won't pay any attention to trying to learn to fetch. I have tought him a few other tricks (sit, stay, play dead, lay, only eats when I tell him too...ect) so he's not stubborn or stupid. So how do I get him to fetch? I can't believe he's forgotten what evolution was supposed to make him love