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Does he chew the water bottle spout at all? It could be he is taking in a lot of fluid. If he does chew the bottle I would suggest the bowl only.
I would up the seed mix part of the diet so he is having more dry grain. Do that gradually. Keep up the veg however only a small fingernail sized portion.
Hamsters do pass a certain amount of soft poo which they re-eat and this passes through again where they extract all the nutrients from the food. It is softer than the normal black poo. This is usually dry very quickly but young hamsters may not recognise the softer poo at first that should be re-eaten.
Keep a close eye on him and see the vets if you are in any way concerned.