I mean if he don't pay attention whether his clothes getting najis or he don't thinks about najasat. He just pay very little attention on taharat like if he sees something like blood he washes away but invisible najasat which is ritual impurity he doesn't pay attention
Is his prayer valid?

If he knew there was najasaat or had a doubt there was najasaat on his clothing before prayer and did not investigate to remove it then he would need to repeat that prayer as Qadha. However, if these thoughts never entered his mind and he just prayed than there is no need for qadha.