I'm just going to throw out there what to maybe expect when you weigh next week. My ball is underweight himself (telltale very triangular body shape!) I weighed before the first feeding - 146 grams, about 6-7 months old estimated. I weighed the mouse at 21 grams, a hair over the 15% weight recommendation. When I weighed him the next week I was ecstatic to see that he weighed 154 grams!

....aaand then he pooped on feeding day, 5 days later, and I weighed him again. 145 grams. DOWN one from the week before!

I'm not too worried about it unless there's a trend, but don't panic if there's not an immediate weight gain. He's eating, and that's the big thing. My guy is currently on a 5 day schedule, except this week it's 7 because I'm leaving for Thanksgiving break. If he doesn't start gaining I'll eek it up to a 4 day schedule. Because you don't know what was going on before you got him it will take a few weeks of feedings to see a trend. If he hasn't pooped in a few weeks, for instance, when he DOES poop it might seem that he's dropped dramatically, even though he is up from what his "normal" poop free weight would be.

Just to give you an idea. Give it a couple of feedings to really see the trend.