It sounds as if he is suffering with impaction. This can happen when they get old and the muscles lose tone.
It is a good idea to see a vet to check that is actually what the issue is though.
A good hay based diet can help. They should also have one cup of veg and one tablespoon of plain guinea pig pellets per day
You mention ‘grain’ on your other post. What do you mean by this - do you mean plain guinea pig pellets? Piggies should not eat grain
If he needs a recovery feed due to not eating enough hay, then that is fine and absolutely the right thing to do - it provides the high fibre content that he needs, but I would cut out the banana. The high sugar content of fruit will not help him.
The guides below may help you
Boar Care: Bits, Bums & Baths
Impaction - How To Help Your Guinea Pig.
Complete Syringe Feeding Guide
Weight - Monitoring and Management