My son is the same age and also loves Thomas, Mickey Mouse (Donald Duck), and balls.
Some suggestions I thought of since you indicated you may want some for things OTHER than Thomas trains and Geo:
~ Puzzles (Melissa and Doug is a good brand)
~ Fire truck
~ Radio Flyer: wagon, tricycle, riding turtle, etc.
~ sled (in case it ever snows enough!)
~ art supplies: finger paints, colored pencils, markers, glue, blunt tip scissors, crayons and coloring books
~ bath tub toys: squirty ones, floaty ones, bath crayons, foamy soap (comes out like shaving cream), color tablets (they come in primary colors to tint the water, and you can teach the color wheel! blue-and-yellow-make-green!)
~ Books or stickers - my son loves them both
~ dress-up play clothes: doctor/surgeon, farmer, fireman, policeman, Halloween costumes!
~ Lincoln Logs, Tinker Toys, Mega Bloks, Legos
~ tool bench: tool belt, hard hat, hammer, drill, screwdriver, wrench, plyers, saw, tool box, etc...
~ Play Doh and any accessories
~ anything he can build with, ride on, crawl under, climb on, match up, pull apart or put together
~ CDs or DVDs - I checked out a Thomas CD from the library and my son LOVES it - we have three Thomas DVDs he just started watching. Bob and Larry CD is a BIG hit as well, so I might get him one of their DVDs, too.
Hope that helps! Have a MERRY CHRISTMAS!!!