Could it be mono?

My boyfriend got a sore throat about a week and a half ago and had some trouble swallowing so went to a clinic and they told him that he had strep throat and gave him some antibiotics. A few days later after not much improvement, he went to a specialist who, because his right tonsil was swollen, decided to test for mono (this result will be relayed to him possibly tomorrow) and gave him stronger antibiotics. The last few days there has been improvements, and today he was told that he tested negative for strep throat. Could this possibly be mono or could there be another cause of this sore throat? I currently have no symptoms of mono at all and to the best of my knowledge I have never had it. Neither of us has been in contact with anyone else in about 5+ years and we have been together for a few months now, so if one of us had mono, the other would probably be affected as well.

Thank you for the help

Note: My boyfriend is also taking Humira and Methotrexate for Rheumatoid Arthritis.