It depends on his real ability and the extent of it. Every Muslim is responsible to perform his worship in the proper way as far as he can. If there is a real inability, then what ever is possible for him is obligatory on him. If he can not stand at all, then he can pray while sitting, but if he can stand for a while then unable to continue standing, then he has to stand for Takbiratul Ehraam and what ever he can, then when he feels unable to continue, he can sit. If he can sit and pray, he can not then pray lying on bed unless he reaches to a stage that he can not sit. Standing is must as far as it is possible even part of it. Same with sitting for those who are unable to stand, then lying on bed for those who can not sit either at all, so they sit, or some times, so they sit according to their present ability.
Rokoo' and Sojood is performed also according to the extent of the ability.