my son peak up any word like maa first after somedays it is

i want to know. Whenever my son peak up any word like maa first after somedays it is clear but after sometimes it becomes aa again means so unclear. I tell repeatedly to correct but he cant. This is just an example. It hapens to dadu also like he says aadu or something else. But sometimes he says correctly. I want to know why this inconsistency happens. Any suggesstion so i can help him. He is almost 4 yrs under spectrum. Thank u all.