Eyes have been open for 4 days, habitat and diet change questions.

[Falkor]

Hello everyone! I have read a million things about the "next step" in housing and diet, and they all contradict each other... so I'm hoping to just run with the information that I get here since we're 4 days into my baby male's eyes being open.

He is an Eastern Grey and he's about 78-80 grams, which I feel is underweight based on how old he is (4.5 to 5 weeks?). I am feeding him Esbilac Puppy Powder (with the added Probiotics banner at the top) formula and have rodent blocks in his box (that he is NOT interested in yet). He eats right at 7% of his weight in formula (about 5.5 cc's) and he's a pig - I don't let him eat too fast, but I am afraid to give him more at each feeding even though I need to fatten him up! He eats about every 4 hours (still at night as well). Otherwise seems healthy, energetic, and very excited to eat.

After he eats recently (since eyes have opened), he wants to play and explore before he goes back to sleep, so I feel like I need to change his habitat/chew "toys". How much room should I give him? We already built a pre-release cage for him, but it's made of wire mesh sides/wood frame and I'm afraid of him climbing the sides and then falling at this age. I have read that nuts (or any other food) are not good to introduce quite yet until he learns to eat his rodent blocks.

Should I already be putting hardwood branches/limbs in the box with him? He has gained an interest in chewing and I don't want to neglect him of the proper teething items.

To summarize my blabbing:

1. Is he too underweight? If so, how else do I fatten him up (eats 7% formula - 5.5mL - every 4 hours and almost none of his rodent blocks)?
2. How big does his box need to be at this age (curiosity growing)?
3. Should I provide anything to eat besides rodent blocks & formula yet?
4. What else is safe/necessary to have in his box at his age (chewing, etc.)?

I still have a heating pad (set to Low) under half of his cardboard box with bedding - he seems to still need help producing heat as his body becomes slightly colder when we take him out to feed and shortly play. I wrap him in a warm blanket in my hand while he eats warm formula immediately after taking him out of box with heating pad.

Thank you all in advance!