Allah made salah obligatory on every sane Muslim to be observed on its appointed time. For this purpose, Allah facilitated it and allowed various ways to perform it in compliance with one’s state.
As regards the questioner’s case, as long as he is advised by a trustworthy doctor not to sit for long period, nor to bend or prostrate; he can perform the Prayer standing. That is to say; keep standing and recite Al-Fatihah and the other surah as he can, then say Takbir (Allah Akbar) and bend a little for ruku’ and more for sujud. If he cannot bend his back, he should bend the neck for ruku’ a little and more for sujud. If he cannot bend his head forward, he should say takbir and recite Qur’an, and intend in his heart the standing, bowing, rising, prostration, rising, sitting between the two prostrations and sitting to recite the tashahhud, and he should recite all the prescribed adhkar. As for what some sick people do of gesturing with the finger, there is no basis for that.
If he is unable to stand, he can perform it while lying on his right side, in the above mentioned description.
For more details on this issue, the following is a fatwa by the Standing Committee for Islamic Research and Ifta’, concerning similar question and they said,
Prayer of the sick person:
1.The sick person must pray standing as much as he can.
2. If he cannot stand, then he may pray sitting, but it is better for him to sit crossed legged at the times when one normally stands during prayer.
3. If he is unable to pray sitting, he may pray lying on his side facing towards the qiblah. It is mustahabb for him to lie on his right side.
4. If he is unable to pray on his side, he may pray lying on his back with his feet towards the qiblah.
5. If a person is able to stand but he is unable to bow or prostrate, then standing is not waived in his case; rather he should pray standing, and lean forward for ruku`, then sit and lean forward for sujud.
6. If there is some sickness in his eye, and a trustworthy doctor has told him, “if you pray lying on your back then it will heal, otherwise it will not,” then he may pray lying on his back.
7. If he is unable to bow and prostrate, he may lean forward for them, and he should lean lower for prostration than for bowing.
8. If he is unable to prostrate only, then he should bow and lean forward for prostration.
9. If he cannot bend his back, he should bend his neck, and if his back is bent so that it is as if he is bowing, then when he wants to bow he should lean forward a little, and make his face as near to the ground as he can when prostrating.
10. If he cannot tilt his head forward, he should say takbir and recite Qur’an, and intend in his heart the standing, bowing, rising, prostration, rising, sitting between the two prostrations and sitting to recite the tashahhud, and he should recite all the prescribed adhkar. As for what some sick people do of gesturing with the finger, there is no basis for it.
11. If the sick person becomes able, during his prayer, to do something that he was unable to do, such as standing, sitting, bowing, prostrating or leaning forward, he should start to do that and continue from what he has already done of his prayer.
12. If the sick person or anyone else sleeps and misses a prayer, or he forgets it, he must pray it when he wakes up or when he remembers it, and it is not permissible for him to leave it until the time for the same prayer begins (the next day) and then pray it.
13. It is not permissible to stop praying under any circumstances; rather every accountable person must be keen to pray in all circumstances, whether he is healthy or sick. This is because Prayer is the pillar of the faith and the most important obligation after the twin testimony of faith. So, it is not permissible for the Muslim to fail to offer an obligatory prayer until the time for it has ended, even if he is sick, so long as he is of sound mind. Rather he must offer it at the proper time according to what he is able to do on the basis of the details mentioned above. As for what some sick people do of delaying prayers until they recover from their sickness, this is something that is not permitted and there is no basis for it in sharee’ah.
14. If it is too hard for the sick person to offer every prayer on time, then he may join Zuhr and ‘Asr, and Maghrib and ‘Isha’, at the time of the earlier or later prayer, according to what is easy for him. If he wishes he may join ‘Asr with Zuhr at the time of Zuhr, or if he wishes he may delay Zuhr and join it with ‘Asr; if he wishes he may join ‘Isha’ with Maghrib at the time of Maghrib, or he may delay Maghrib and join it with ‘Isha’. As for Fajr, it cannot be joined with the prayer before it or after it, because its time is separate from the times of the prayers that come before and after it.