We saw adorable costumes at Target. They had peas in a pod, a banana, and a chili pepper. I don't know if they were the right size for your little guy though.
For my son's first halloween, he went as Mario. Red long-sleeved shirt, blue-jean overalls and a red hat all store-bought. We took a bit of white patch, cut it in a circle and stitched a red M to it then stitched it to the cap. Here's a pic. I'm Princess Peach in this pic. A very chubby, brownish hair Peach.
Otherwise, they have some really simple patterns at the fabric store, like See 'n Sew costumes and stuff.
Just a few ideas. Hope this helps and good luck!!! We're working on our costumes this weekend as well. My DS is going as a sheep this year and I'm going as a Settler. Should be fun.
LM