First: DISCLAIMER THAT I AM NOT AN EXPERT BY ANY MEANS!
My experience comes from wild bunnies, so hopefully someone on the forum with experience with domestic babies will hop on here quickly.
Babies get skinny fast, but fill up just as fast!
Like bam said, you need to keep him nourished and warm.
When my Pete was that tiny we lost the rest of his litter within the first 48hrs of them being separated from their mom. We made a formula for him from dry kitten milk, pedialite, water, and bunny vitamins. The pedialite keeps them from getting dehydrated too quickly and you can get it clear/unflavored. Don’t put the baby on it’s back during feedings or you risk aspirating him/her, instead use a dropper and go slow. I suggest to gently rub it’s tummy a little, periodically during and after feeding. They fill up fast and you don’t want him over-full and uncomfortable. The frequent tummy rubs will also help you to feel when the tummy is firm and full, so you know when that feeding is done.
Also, after you’re done feeding, be sure to either take a warm damp qtip or wet the tip of your finger and rub it’s bottom. The food and tummy rubs help with getting the digestion going, but the stimulation at their bottom when they’re very tiny helps them to poop. It also reinforces to them that grooming is a good thing. Doing the same thing to his/her little face is reassuring to them and also ensures that the formula doesn’t dry around their mouth and nose.
I’ll be sending good vibes to you and your bunny-baby!