Praise be to Allah.
If he cannot prostrate from the time when the imam says the takbeer until he rises, then he should prostrate with the imam, then if it becomes too hard for him to continue prostrating, he may rise a little but stay as close as possible to the ground, and this will not mean that he is differing from his imam. If he cannot prostrate on the ground, then he may prostrate as close as he can to the ground. End quote.
Shaykh Muhammad ibn Ibraaheem (may Allaah have mercy on him).