That is the stuff,

here in the UK it is only avaiable from the vet and is quite expensive.

Sage really needs to see the vet, if you leave it it could be too late for him. Can you not beg your mum to borrow the money?

Or as a last resort see if there are any rodent rescuers who would take him off you and get him treated, or if they would do that at the shelter.

Leaving him untreated is simply unacceptable, you should not have bought him if you couldn't pay for vet bills.