Our 7 month old boxer has stopped eating, and is very lethargic?
This started yesterday?

That sounds just like the symptoms my old Lab has had several times this year. I could suggest it is pnuemonia like her, but what I will actually say is get it to the vet in the morning. Let the vet determine the real problem, and an effective remedy.

our 7 month old boxer has stopped eating, and is very lethargic?
this started yesterday?

Has your puppy been inoculated for parvo?

Vaccination history? Exposure to other dogs? Diet? Neutered?