How long have you had him? When I brought my female syrian home, she was a bit hit and miss with her food while she was settling in - she has a great appetite now though!
If he is still bright and active, I would just keep an eye on things for the moment. Maybe cleaning his nest out affected him slightly? My youngest robo took very badly to me cleaning out her nest for the first time. She acted very strangely for a few days afterwards.
I don't know if weather can affect a hamsters appetite like it can with dogs but it's been very warm and stuffy where I am (in between the crazy thunder, lightening and torrential rain
).