Losing weight....

I weigh my rats at home and I'm extremely worried about Milo. He's been losing weight over the course of a week, but otherwise there is nothing obvious wrong with him. I would take him to the vets, but they probably will not be able to see anything as I have completely checked him over and cannot find anything on him like tumours, his breathing is fine, and his pee is normal too.

Friday (last week): 680g
Saturday: 678g
Sunday: 670g
Monday: 665g
Tuesday: 660g
Wednesday: 658g
Thursday: 650g
Today: 645g

That's 680 grams to 645 grams in one week.

Help guys?

is he on any medicine or has he been within the past few weeks/month?
it is worth taking him to the vet if he is losing that much weight, he could have something internal going on that a vet is more likely to feel than you (sorry if that sounds rude). you can rarely feel internal lumps at home, at the vet can tell you exactly where they are and what size usually just by having a feel about. i don't mean to scare you and this might not be what is wrong.

I would definitely take him to the vets, that's alot of weight to lose in a week, as hammock maker says it could be something internal that a vet would be able to find, i really hope you boy picks up soon xxx

How is he doing? xx

He is still losing weight and I can feel his spine at some parts of his body, however, the vet can't feel anything. He hasn't been on medicine at all, the vet gave him a steroid shot to encourage him to eat but it's not had any good effect on him. The vet had a long feel inside him for anything that shouldn't be there, and yesterday he went in for an x ray which showed (again) nothing.