nooboo,
My Nibbles when he was around 3 months old still only weighed in at 2oz. He slowely put on weight and then only around 6 months plus did he gain. his fur also didn't really grow all that much till after 6 months (Long-haired).
I think some are just genetically going to be smaller / lighter. As long as you keep an eye on things and Ponder is eating plenty and acts ok then there may be nothing to worry about.
As for peeing on his food - do you put his food in a bowl or scatter it on the ground? Scattering it on the ground (some of it) can be a good thing sometimes as it will encourage natural forageing and he's not going to pee on every little seed he finds scattered around! So for a while maybe scatter his food in several little piles throughout his cage and see how that goes.
Just a suggestion
