[souffle]

I would just wait and see how he goes when he is getting more wheel excercise. Cut out his 'sweet' treats like yoggy drops and people food and also only feed every second day so that encourages him to eat all types of food rather that just the sunflower seeds and fatty things.
He may just be a big made hamster and there is a difference between a big hamster and a fat hamster. Fat hamsters have folds of skin and feel all squidgy where as a big hamster will feel solid and firm.