7 year old is refusing to eat (yes I've read the stickies)

My 7 year old corn is a rescue. He hasn't eaten in 3 weeks and while I know this isn't a very long time for an adult corn, he's already incredibly skinny. I'm feeling uncomfortable with him not eating at all. Last week he shed so I assumed that was the reason he didn't take his first mouse. Now I tried feeding him again today. He opened his mouth and gnawed on the mouse very softly, but after about 20 seconds of this he backed off and is totally ignoring the mouse. The mouse isn't too big for him.. he's a little over 3 feet and the mouse is just a little larger than a fuzzy (about the same size as his tiny skinny body). I'm not sure how exactly and how often his old owner fed him. I tried emailing her about 2 weeks ago and have not gotten a reply.

And I really do not have the money to go out a buy a ton of drops, different size mice, etc. if I'm not sure it will make him eat. I'm really stressing out and I would appreciate any help...
Yes I've already ready all the sticky information and many other threads on similar topics. I've tried braining the mouse, cutting slits, leaving it in his cage, wiggling it with tongs, etc. I don't have any chicken broth and I literally have $1 right at the moment so unless I go beg my mom or friends I can't afford to go buy a ton of products that may or may not work