What Causes Air Discharge From Penis While Urinating Post Treatment For Prostate Problem?

this is a serious question, my father has air come out of his penis when urinating.....sometimes for last week or so...is this a symptom of something? He is 85 years old and had his penis (drilled out due to oversized prostrate) worked on about 10 months ago...no other issues. SorryI am not good with the correct terminology. I live in Colorado and he lives in Ohio?? Thanks for any advise.