It may take up to 48 hours for mom's milk to come in. Hang an additional bottle of 50/50 water/apple juice or water/unsweetened cranberry juice. This will hopefully stimulate her to drink more, which should help with production.
I would only supplement if the baby starts losing weight or gets inactive again like you described. The more the baby suckles, the more milk production is encouraged. You should weigh the baby on a gram scale twice daily for the first few days, then once daily for the first few weeks.
IF you have to hand raise the baby (and you usually don't), you'll need to feed every 2 hours. The most commonly used formula is goat's milk based and is here:
1 can goats milk (in canned milk section at grocery store, evaporated milk can be used if goats milk can't be found)
1 can water
1 tablespoon dry rice baby cereal
1 tablespoon plain active culture yogurt
2 drops of light Karo syrup
There is a page on hand rearing kits here:
I would also get some colostrum supplement if you wind up feeding him, and use it as soon as possible. I just hand raised two kits who were orphaned at a week old and one got so far as weaning and then just died due to an imbalance of gut bacteria because he didn't have mom's milk to establish that balance as a baby.
Also make sure they're in a baby safe cage. Babies can escape from wiring much larger than 1 1/2" x 1". Give mom some time alone - she's probably just got the first time mom nerves going on.
Here's another page on breeding:
Best of luck!