Schnauzer losing weight

(Nada Glavan)

My 2 year old black miniature has suddenly lost a good amount of weight, his once muscular body has become quite sinewy with skin that I can pull away like a pup's.
This has all happened rather quickly as I recall just thinking recently how solid and muscular he was- not fat! without an ounce of loose flesh. he seems healthy with a shiny coat.
He does spend his days playing and chasing our new 7 month old pup so he does exert a lot of energy but that has been the same for the past few months.
He is regularly wormed, heart wormed and has all his injections up to date.
Anu ideas?