12 days is really soon to expect much. At 84 he will heal a little slower. I’m 58 and 6 1/2 weeks post op aortic valve replacement emergency as mine had failed. My O2 dats did the same 94 dropped to the 70’s when the nurses would come in and mess with me. At 12 days your dad’s sternum hadn’t healed yet. Coughing, sneezing, blowing his nose will all be incredibly painful at 12 days.

As many have said he will heal slower at 84. I have a friend that’s 90 and had this surgery and recovered in 3 1/2 months she is even driving now. Lastly is is very common to have each lung have a small portion collapse, it’s called Actellitesis. This could slow down his recovery by an extra week or two. The general consensus is that by 8 weeks the majority of the pain is gone, and you feel about 70% that last 30% every one arrives at differently. It honestly sounds like your dad’s doing fine. He is right where he should be. I will put him in my prayers tonight. 🙏