[cypher]

Have you checked his teeth?
If you're not confident doing that yourself then get your vet to do it, they can become overgrown or sometimes get damaged which can prevent them from eating hard foods but the teeth continue to grow so it's very important to have them clipped if need be.
In the mean time you can feed things like soaked mix, safe baby food, & porridge but do check them as soon as possible.
Sometimes as they get older they don't seem to enjoy hard foods so much & may need soft foods but it is still important to keep an eye on their teeth.