It's really hard for me to determine low end normal by feel. I'm good with really thin and with fat , everything else I find more difficult. I've palpated a lot of birds, it could be I just suck at it
Remember his pectoral isn't developed yet .
You can wet his chest down and part feathers that makes it more obvious if really thin. And weigh
Also young ones take more naps .
This young still in rapid development stage . Nutrition is important. Protein is needed for growth. You could offer a little boiled or scrambled egg. You can pick up baby bird formula and mix a little thicker and offer in separate dish, see if interested in eating it as a support thing ...I would also start offer soaked and softened pellets , with ir without a little formula mixed in . If he is eight weeks old seeds alone are cutting it for the nutrition he needs . He may also be so young that he isn't able to crack and eat enough seeds for himself yet .
Does he poop often with good fecal amount in poop?
Being on the floor is stressful, I would raise the cage up on a table , add at least one more perch and a toy by it. Low grade stress can keep him from getting deeper sleep during naps .
But newly weaned birds are more at risk from health stuff , because of how their immunity is changing at this age . So any concerns I always recommend vet check .