ANSWER:

He has got the ruku and the rakah as long as he caught the imam in his ruku position even if he didn’t get to say the tasbeeh.

We know that whoever gets the ruku has got the rakaa. But if someone gets imam surely at the ruku position but before the follower finishes ruku tasbih like subhana rabbiul azim once, the imam starts rising from the ruku. In this case, has the followet got the ruku or he has to repeat the raka after imam finishes his prayer.