I'm looking for a teen paperback novel that's at least 20 years old. An alien ship crashes & the only survivor was a teenage boy. I think the captain of the ship was his uncle or father, & had the boy get into an emergency pod. He had two hearts, and telepathic abilities. He required a higher oxygen content at night, but could manage without the extra oxygen during the day. He befriends two human teenagers - I think it was a boy & a girl. He, his friends, and (I think) a military group end up thwarting an alien invasion. The aliens took over human bodies. He could recognize the aliens by their thought patterns or a feeling, & humans could recognize them by an odd look or possibly shimmer in their eyes. It ends with the boy returning to his home planet, & his friend gave him a packet of grass seeds for his home planet, because it had almost no vegetation.
The book is The Boy Who Saved Earth by Jim Slater.
He came from another planet. Unfortunately he did not come alone… When evil alien forces attack Earth and intend to destroy it, Marcou – the 14-year old from outer space – must use his telepathic powers and ultra ray secret weapon to try to stop them.
One of the Goodreads reviewers says:
I remember reading this as a kid back in the 90s. Although I don't remember much about the plot anymore (except that he saved Earth, of course), I remember enjoying reading this book and giving me a taste of what sci-fi has to offer and for that I always appreciate this book. This is probably out of print now but I was prompted to look up this book here on Goodreads after seeing a picture of two hearts in an alien somewhere on the web because the alien boy has two hearts! I also, for some reason, have never forgotten the last line of this book which is "grass might grow on Gundra (the home planet of the boy)."