348 grams is pretty small. I'm guessing that he's not full grown yet. Do you have any idea on his age? It could also be that he is just small, chins come in all shapes and sizes.
On a separate note, is that smashed poop on the yellow thing in the picture? If it is, his poop shouldn't be that soft. You might want to try and give him just lots of hay for a day or two (no pellets or treats of any kind) and see if it gets better. Soft poop can be caused by lots of things, most common for new chins is stress from the move, and too many treats. If he doesn't go back to harder pellet-like poop after a day or two of just hay, you might need to see a vet.