Hi and welcome to EBN. Sorry to hear about your new bully. I don't have first hand experience, but I'm thinking that having pneumonia at an early age doesn't necessarily mean that he'll develop it again later. Their immune systems are not as strong and still developing, and along with the cold weather, they can catch anything easily. Hopefully you can have him back once he's better as well, I don't see why they wouldn't. You've showed your attentiveness by seeking care when you realized that something wasn't right. To me, that's one of the signs of good parenting.
Please give hugs and kisses to your little one from all of us. We'll keep both of you in our thoughts and prayers!