I have done all my kids cakes and it is a lot of fun. Now a day there is a lot out there for you to come up with a very cute cake.
Get cake mix and bake few round cakes and square cakes, put themin a big plate or hard card board they sell them at Jo-Ann Fabrics or Michaels. You can do your own butter cream icing or buy it, Wilton’s is pretty good. After you put the icing then start creating. At the dollar store you can find the little plastic horses in a bag you can use those or if your son have any wash them and use them. If you want to create grass use coconut flakes and color it green with food color. After you start you want to continue and trust me you will be surprise with all you can do.
Good luck and if any other tips write my back,