Hi, W. --
This is the first of an almost endless round of school projects that will threaten to take over your life during the course of your son's academic career -- and you already have three times all the stresses and all the joys of motherhood in your life --so do yourself a BIG favor and start thinking along the lines of "easy" and "fun" rather than "grand" and "amazing."
Did I just strike a chord? I'm expecting I might have, because your request has just a hint of the ambitious perfectionist's desperation in it! Believe me, I know all about that feeling from first-hand experience! But I'm working on it....LOL !!!!!
The easiest way to make a model without using clay or play dough is to use paper. Go to the dollar store and buy a coloring book. Have your son color the dinosaurs. You should also fine some great scenery in the book -- mountains and trees, etc. If you're lucky, you'll even find a volcano, which your son will probably love. Take a shoebox or a larger box and start by letting your son glue the mountains and trees to the back of the box. To make the dinosaur stand up in the foreground, cut a strip of thin cardboard or construction paper and fold it in half. Glue the bottom of the paper strip to the box. Glue the dinosaur to the part of the paper that stands up in the air. Viola! Instant standing dinosaur.
That's all that's really required, but your son can add more as (and if) time allows. Have him color a piece of paper blue and glue it to the bottom of the box to create a lake. Or just use construction paper. Small green pieces of paper paper colored green can be blades of grass. Make them stand up around the water the same way you folded a piece of paper to make the dinosaur stand up.
Drape some brown twine or yarn from the top of the box to create vines. It's easy to cut leaves from green construction paper and thread them onto your "vines" before looping from the top of the box.
If you pick up all the supplies in one quick shopping trip, and have your son color all the pieces over the next couple of days, it should only take about an hour or so to put the model together.
Good luck!