Yep, I'd be going the worming route before you hit the panic button. Especially if it's been eating wildlife. Our cat used to eat lizards when she was younger and she had to have some extra worming thing because of a parasite in the lizards. Anyway, I think it could be something simple like that, but I really do think it would be best for him to be assessed by a vet. They might go gently with the fees given that he's a stray/charity case - it's worth ringing and asking anyway.