Are the scales digital or anagloge? Cause if they are analouge then it may not be 1kg it could less.

I would actually keep an eye on him first. Weigh him once a week at the same time of day(pref morning before brekkie) while naked so you get a proper reading. Write down what actually goes in his mouth each day. You'd be surprised at how much they do actually eat when you keep a record of it. Then after say 2-3 weeks if his weight hasnt changed or he has gained i wouldnt bother with the checkup. If he's lost more then certainly get it checked. If he is happy in every other way i would say he is fine.