Since men have less breast tissue than women,breast cancer is not that common,but even though it's rare,it can unfortunately happen particularly in older men.
The pain and swelling you are describing,could be many things and it's not necessarily something serious.It could be due to infection,a pectoral muscle inflammation or strain,or maybe Gynecomastia,which is a condition that usually occurs in young boys due to hormonal imbalance.However it can also happen in adult men.The symptoms usually present with swelling in one or both breasts,and a small lump might be present beneath the nipple.
It's really not easy to say for sure what is going on,but it's a good thing that soon you'll be seeing your doctor who will be able to evaluate your condition and perhaps order a blood test to check your hormone levels, and a mammogram or Ultrasound,especially if a lump is present beneath your nipple.
Make sure to let your doctor know exactly what type of medication you are taking,( If any) because some type of supplements or drugs and also excess alcohol could cause this condition.
All the best to you and your wife and soon to be born baby! :)
Sorry about the punctuation on that post, I'm not used to using my phone to post on message boards. And thank you for the reply