Annoying though it is and it probably ruins his clothes after a while, it is a harmless habit which soothes him just like sucking his thumb or carrying round a security blanket would. My son used to do this too. The collars of his
t-shirts were always damp, his cuffs stretched and chewed....and there was the corner of his duvet cover which was always scrunched, twisted and damp!! Obviously helped him relax to sleep to be sucking on it! I just ignored it.