Im not really experienced but didnt want to read n run...
It might be a comfort thing....
My little sister used to pull the sheet in her cot and suck that.... (so much so that my dad's mum saw the state of the cot and accused my mum of being a bad mum)....turns out my sister needed it....so growing up she had a "sucker" which was always an old pillowcase or my mum's t-shirt and she used to suck it when she went sleep but she also used to do it when watching tv or relaxing....she grew out of it around 9/10 years old...but my mum preferred it to her sucking her thumb as it never altered her teeth....
If you dont mind him doing it, give him something like a old t shirt to suck rather than his clothes....i dont know cos at his age he wont understand "No" but maybe if everytime you see him sucking his clothes, stop him and offer him the alternative....(maybe the blanket isnt the right texture to suck) if you dont want him to do it then i dont know as it may be a comfort thing....or he may be teething...
Good luck x