Hi there! I've owned and worked with ferrets before myself. They are truly a joy. I'm going to say to keep up with the treatments that your local vet has provided you with. He is most-likely still in the recovery stage. It's just like us; when we are sick, we don't necessarily want to be super playful/full of energy/etc.. As for the getting another ferret; it isn't necessary, unless you simply just want another. As long as you are providing it the necessary nutrition and care it needs along with spending time with him, he's going to be fine. A lot of people say they want to get their male ferret a "mate". Well, even if you get him a female ferret, she's going to be fixed. All retail female ferrets are sold already spayed because they absolutely have to be bred when they go into heat or they will die. Marshall is the only company in the United States that legally breeds ferrets I believe, as it was outlawed due to the conditions of a female not being bred in the care of someone oblivious to the conditions.
In other words, keep giving him love and attention. Wash your hands before and after every handling of him. Be careful with your other pets, especially the dogs. Keep up the treatment and in a week or two, he should be acting just like a ferret. Good luck :)
Myself -- Vet Tech