Hamster lost weight but still energetic and eating

[Ria P]

The lump on his abdomen, is it near his scent gland?

I take it that you've checked his teeth?

He may be hoarding most of the food instead of eating it. Personally, i'd buy a better quality food than petsathome and give a daily pinch of brown linseeds for his fur. Weight loss like that at only a year old is concerning. I'd check his urine with a diastick to see if anything shows up or get him checked by a hamster knowledgable vet.

Sorry i can't be more helpful but weight loss can have so many causes.