Is he doing ok now? Being hunched over can be a sign he's in pain. Just keep an eye on things and feed him whatever he fancies
As well as regular hamster food. If he's older they can start to have difficulty eating if they have toothache or the front teeth get a bit long so try and have a look at his front teeth when he yawns next time. Sometimes they need some supplements or some soft food like baby food when they're older and have teeth problems but it's important to put hard food out as well to help keep their teeth trim (plus they like hoarding it). 4 months baby food is usually ok (ie without spices, onion or tomato). I used to put it in a little lid from the baby food jar and our hammy loved licking it up. Pumpkin and chicken was his favourite.
Hemp seeds are also full of just about every nutrient there is so I used to put those out in a little baby food lid every few days too.