I suffer from some chronic pain issues, which occasionally make it impossible or very uncomfortable for me to get down and play with my son, who is just over 2 years old.
My wife has also had some issues, recently. She sprained her foot during her last trimester, and just delivered our second son and has quite a few tender spots.
The solution we use with my son is to simply tell him that we hurt. Right now, the language we use is, "I can't play right now, because I have an owie. My neck is owie, and my back is owie." I have told him that, verbatim, several times before. He understood very well the first time I told him this. He does act disappointed, but he knows it's not because I have a problem with him. I think on a few occasions I even made that explicit, and said, "Papa wants to play, but I can't because of my owies."
To make it easier on him, I try to do other things with him, such as reading a book while sharing a seat. Other times I try to suggest more playful activities that I know I can physically handle--that is, ones that let me sit somewhere comfortably and interact with him. Some activities like this are Play-Doh or drawing. Often, it's enough that I'm there with him, that I don't actually have to do all that much.
To sum up, my solution is really two parts:
- Just be frank with your son, using language that's at his level. Your 3-year-old may understand more complex phrasing than just saying you having an "owie" on your back.
- Suggest other activities for him to engage in, preferably those where you have some involvement.
I think this will help with other possible reasons for not being able to play with him. For instance, my son quickly adapted to, "Mama can't play right now, she's feeding the baby." He understands that sometimes we're not available.