If you are not able to pray standing or sitting as you are bed ridden, you can pray while lying in bed through gestures and motions.  In other words, he should pray through gestures and motions of head and hands the two positions of Ruku and Sujud; lowering a little for Ruku and slightly more for Sujud. If that is also not possible, he should simply pray as best as he can in his mind. Allah says, “Practice mindfulness of Allah as best as you can.”

Almighty Allah knows best.