I don't think the scale can be right if he's 3 months old
The Syrian weight range is usually between 100g and 300g. He doesn't look undersized or undernourished to me. I use these flatbed digital scales. They're not too expensive and accurate unless the battery is going down! They make it easy as well as you just put a treat like cucumber in the middle, then zero the scales so the cucumber weight is discounted, and the hamster just walks on and sits eating the treat while you read the scales.
Scales
He looks perfectly healthy to me