If you wanted to achieve a Thomas theme without having to spend a fortune, I would suggest doing a Thomas bedskirt and Thomas pillowcases or shams. Add a solid colored sheet set and a solid colored comforter from any department store in a coordinating color. Then, when he outgrows Thomas the Train, you would only have to switch out the pillowcases and bedskirt.
Below are the items I can make. (Keep in mind that the Thomas fabric is fairly expensive compared to other fabrics.)
Queen Bedskirt - $30
Queen Sheet Set (standard solid colored sheets with Thomas trim added to header of top sheet) - $38
Pillow Shams (2) - $40
Pillow Cases (2) - $20
Custom Queen Quilt - $150
I can also do throw pillows, curtains, etc. Just let me know if you need more info on those.
Here are some fabric choices to give you some ideas....
