بسم الله الرحمن الرحيم
(1) He should join in Zuhr and Isha as it shall be nafl . Further he will also get the reward of jamat.
He should not join in Fajr and Asr because it is makrooh to offer nafl salah after these two salahs, and he should not join in Maghrib for there is no three rakah in nafl.
(2) He is musafir, and has offered his fardh salah then the same the abovementioned ruling shall apply if he joins the jamat.
Allah knows Best!
Darul Uloom Deoband