Saw your signature pop up in a recent thread tonight and I had to check out some more photos of your cute little guy. What a little doll! He looked about 7 weeks to me. How is he doing now?
When we found Little Man, he too was very petite (still is) and I gave him KMR for while to boost him a little bit. He's petite, but very healthy. I strongly recommend it. It's a little pricey. It's cheaper in the powdered form, but don't make too much in advanced, the label says it only keeps for 24 hours. I still have some left in powdered form, if you're anywhere near VA I can give it to you. I personally don't recommend the Hartz kitten milk stuff. I really like the Just Born brand (it's in pink packaging.) Good luck!
The ears mostly give it all away. Shape.. they are still 'cubby' looking. So I don't think much over 5 weeks when those pictures were taken.
Cute booger though!
I use just the powdered KMR. Petsmart sells it cheaper than most pet supply stores. $24 for a normal can at the local petstore, meanwhile $15 at Petsmart. But definately worth the money.
I do use the premade liquid cans of KMR if the kittens are really young and/or malnourished.
A high quality kitten food would be really good for him too. I'd suggest the Royal Canin Babycat until his widdle teefies are bigger. I use that when I wean my bottle babies from the formula, they love it.
You can normally tell by how much they weigh. 1/4 a pound per week of age. (so if he was 4 weeks he would weigh 1 pound) Shelters like to say that their older then what they are so they can adopt them out sooner. Everyone likes the little babies so its easier to adopt them when their so small and cute.
5-6 weeks, at the MOST. He's a pretty tiny baby...I would pick up some KMR and canned kitten food. Keep him in a smaller "safe room" with a shallow water dish, food, toys, sleeping area (a pop can flat lined with a blankie would suit his size just fine), and shallow litter pan (a foil lasagna pan works well when they're this small).
There's always the possibility as others have mentioned that he was malnourished and has not yet up to the size he really should be...thereby, being a bit developmentally-delayed in size and behavior. I do think, however, you basically have a really young baby on your hands.