I'm quite late in replying to your question.
But,your partner? I'm assuming after he went to see his doctor,and was told his muscles between his rib cage were inflamed? I disagree.I am clearly not a doctor,and it's unlike anyone to disagree with an M.D. But,I am!
I personally believe your partner 'may',which I am almost 90% sure he has a condition known as Costochondritis,which is somewhat similar to what his doctor diagnosed him with having.
But,different in a way.....opposite diagnoses actually.
With Costochondritis the persons cartilage,which is located between the rib cage becomes inflamed. Thus causing the intense pain,and inflammation.
The symptoms of Costochondritis are increased pain upon deep inhalation(taking a deep breath),increased pain with certain movements,pain radiates to other areas(upper/lower back,neck,shoulders,arms,etc.),increased pain when laying flat,etc.There are other symptoms associated with Costochondritis that I didn't list.
There is no muscle between the ribs.....only cartilage! Fact! So,that is why I seriosuly think your partner 'may' have this medical condition....Costochondritis.
I suggest you and your partner go back to the same doctor,and ask about this medical condition,and ask if this may apply to your partner.But,do keep in mind.May doctors don't like to be proven wrong.My opinion anyways.There really is no treatment for this medical condition other than anti inflammatory pain medications,and alternating cool/warm moist compresses.Many times corticosteroids help with the inflammation which in turn helps rid the pain.But,when it comes to steroids? There always runs a risks,such as stomach issues,and so forth.This usually applies with long term usage of steroids though,so speak with his/her doctor in reference to steroid treatment.
Hope this helps.
Take care.