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.
He's a little pumpkin! Cute baby!!!