it's good you have a doctor's appointment already because the symptoms you describe your husband is experiencing sound like a umbilical hernia. There is a thin layer of muscle tissue below the navel, that keeps the intestine in place and any tear in this layer can lead to hernia, which is basically a part of the intestine that slid through the muscle tear. These muscle tears happen mostly due to lifting heavy objects, but sometimes other conditions can weaken it, as well.
And since it's growing more and is causing pain, I'm sure that after your doctor confirms what exactly is going on, your husband will probably be scheduled for hernia repair surgery. It's a relatively simple, outpatient procedure that repairs the muscle wall and restores the part of intestine which formed hernia to its place.
Wish you all the best,