Hi!
Your boy needs to see a vet promptly if he is not right. Please switch from the regular once weekly weigh-in to weighing daily at the same time in order to monitor his food intake. 80% or more of the daily food intake is hay, which you cannot control by eye and which is often the first food group that is being affected. If he has lost more than 50g of weight, he should be seen and you should start offering him additional syringe feed to keep his fighting strength up until any vet treatment can kick in fully.
Please don't wait too long; the sooner you see a good vet, the sooner your boy can be treated and the better his chances are that he can make it through and delight you for a goodly while longer!
Complete Syringe Feeding Guide
If you have seen a maggot in the cage, please check him for fly strike around the genitals; if you find signs for it, he needs to see a vet asap as an absolute life and death emergency. Fly strike can be sorted in the early stages but is fatal once the flesh eating maggots develop and do their job. Make sure that you clean out the cage and disinfect well.
Fly Strike
PS: I have moved your thread to the specially monitored Health/Illness section where you will get more experienced answers.