Since he's so young you should be able to do something simple for him. Orange sweat suit. Face paint (or just paint a nose/mouth & whiskers on with eye-liner). You can buy cat ears and tails, though usually they are black.
I made some once when DD was that age and then she changed her mind about her costume. To make ears, use a head band and put triangles on it - you could use fabric or even stiff paper.
For a tail, you can pin one on the back of his shirt or pants (pants might be better for using the toilet). It could be made from scraps of fabric.
But if you aren't "crafty" I would just buy the ears and tail at a craft/fabric store. They are relatively cheap.
Oh, yeah! Another thing you could do easy/cheap is to use a pillow case or make a simple sack and stuff it (if he wanted to be a fat cat) and you could just braid a tail out of yarn. I went as a mouse like this once way back when. But you could do a kitty too. We made paper plate masks ourselves.
I have to figure out now how to make a furry wolf queen costume for DD (7). She is super picky about everything and wants one leg to be black, the rest to be gray. I am thinking footed PJs style, but I have no idea how to make that. Ack! At least she already has something to use as the cape. And we do face paint for fun around here and I found a good design on the internet.
I am a 40 year old unschooling, belly dancing, artist-mama of one almost 8 year old. I just had brain surgery and about it a bit because it's just so surreal.